Fix flaky epics_finder_spec race condition

As per https://gitlab.com/gitlab-org/gitlab/-/issues/321511 we've seen
one of these tests fail. It seems likely the problem is caused by the
use of `X.days.ago` throughout and then filtering with date ranges. The
likely explanation is that when the test is running just before midnight
then `2.days.ago` actually ends up being "3 days ago" from a date
perspective by the time the assertion is made about it.

Even though it's not easy to replicate this exact failure mode and this
flakiness has never happened for this spec before (to my knowledge) in 3
years it still is not a good way to write these kinds of tests as this
is theoretically a problem.

Instead of making all the times relative to "now" (which changes) I've
just made everything relative to some arbitrary fixed time. This works
fine, without any need for freezing time, as none of the methods being
tested actually refer to the current time.

The test is still quite difficult to read due to many different dates
and times and the fact that the examples are separated from the test
setup but I don't think this change makes it less clear.

ee/spec/finders/epics_finder_spec.rb

@@ -7,9 +7,10 @@ RSpec.describe EpicsFinder do
   let_it_be(:search_user) { create(:user) }
   let_it_be(:group) { create(:group, :private) }
   let_it_be(:another_group) { create(:group) }
-  let_it_be(:epic1) { create(:epic, :opened, group: group, title: 'This is awesome epic', created_at: 1.week.ago, end_date: 10.days.ago) }
-  let_it_be(:epic2) { create(:epic, :opened, group: group, created_at: 4.days.ago, author: user, start_date: 2.days.ago, end_date: 3.days.from_now) }
-  let_it_be(:epic3) { create(:epic, :closed, group: group, description: 'not so awesome', start_date: 5.days.ago, end_date: 3.days.ago) }
+  let_it_be(:reference_time) { Time.parse('2020-09-15 01:00') } # Arbitrary time used for time/date range filters
+  let_it_be(:epic1) { create(:epic, :opened, group: group, title: 'This is awesome epic', created_at: 1.week.before(reference_time), end_date: 10.days.before(reference_time)) }
+  let_it_be(:epic2) { create(:epic, :opened, group: group, created_at: 4.days.before(reference_time), author: user, start_date: 2.days.before(reference_time), end_date: 3.days.since(reference_time)) }
+  let_it_be(:epic3) { create(:epic, :closed, group: group, description: 'not so awesome', start_date: 5.days.before(reference_time), end_date: 3.days.before(reference_time)) }
   let_it_be(:epic4) { create(:epic, :closed, group: another_group) }
 
   describe '#execute' do
@@ -73,15 +74,15 @@ RSpec.describe EpicsFinder do
 
         context 'by created_at' do
           it 'returns all epics created before the given date' do
-            expect(epics(created_before: 2.days.ago)).to contain_exactly(epic1, epic2)
+            expect(epics(created_before: 2.days.before(reference_time))).to contain_exactly(epic1, epic2)
           end
 
           it 'returns all epics created after the given date' do
-            expect(epics(created_after: 2.days.ago)).to contain_exactly(epic3)
+            expect(epics(created_after: 2.days.before(reference_time))).to contain_exactly(epic3)
           end
 
           it 'returns all epics created within the given interval' do
-            expect(epics(created_after: 5.days.ago, created_before: 1.day.ago)).to contain_exactly(epic2)
+            expect(epics(created_after: 5.days.before(reference_time), created_before: 1.day.before(reference_time))).to contain_exactly(epic2)
           end
         end
 
@@ -187,8 +188,8 @@ RSpec.describe EpicsFinder do
         context 'by timeframe' do
           it 'returns epics which start in the timeframe' do
             params = {
-              start_date: 2.days.ago.strftime('%Y-%m-%d'),
-              end_date: 1.day.ago.strftime('%Y-%m-%d')
+              start_date: 2.days.before(reference_time).strftime('%Y-%m-%d'),
+              end_date: 1.day.before(reference_time).strftime('%Y-%m-%d')
             }
 
             expect(epics(params)).to contain_exactly(epic2)
@@ -196,8 +197,8 @@ RSpec.describe EpicsFinder do
 
           it 'returns epics which end in the timeframe' do
             params = {
-              start_date: 4.days.ago.strftime('%Y-%m-%d'),
-              end_date: 3.days.ago.strftime('%Y-%m-%d')
+              start_date: 4.days.before(reference_time).strftime('%Y-%m-%d'),
+              end_date: 3.days.before(reference_time).strftime('%Y-%m-%d')
             }
 
             expect(epics(params)).to contain_exactly(epic3)
@@ -205,8 +206,8 @@ RSpec.describe EpicsFinder do
 
           it 'returns epics which start before and end after the timeframe' do
             params = {
-              start_date: 4.days.ago.strftime('%Y-%m-%d'),
-              end_date: 4.days.ago.strftime('%Y-%m-%d')
+              start_date: 4.days.before(reference_time).strftime('%Y-%m-%d'),
+              end_date: 4.days.before(reference_time).strftime('%Y-%m-%d')
             }
 
             expect(epics(params)).to contain_exactly(epic3)
@@ -214,13 +215,13 @@ RSpec.describe EpicsFinder do
 
           describe 'when one of the timeframe params are missing' do
             it 'does not filter by timeframe if start_date is missing' do
-              only_end_date = epics(end_date: 1.year.ago.strftime('%Y-%m-%d'))
+              only_end_date = epics(end_date: 1.year.before(reference_time).strftime('%Y-%m-%d'))
 
               expect(only_end_date).to eq(epics)
             end
 
             it 'does not filter by timeframe if end_date is missing' do
-              only_start_date = epics(start_date: 1.year.from_now.strftime('%Y-%m-%d'))
+              only_start_date = epics(start_date: 1.year.since(reference_time).strftime('%Y-%m-%d'))
 
               expect(only_start_date).to eq(epics)
             end