Loosen TTL expectations in tests

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70444 added some
tests to verify the TTL of sessions, but tests may run slow so the
timing may not be accurate to within a second. We loosen the check to
ensure the value is within 5 seconds.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/341097

spec/support/helpers/session_helpers.rb

@@ -13,7 +13,7 @@ module SessionHelpers
     session_keys = get_session_keys
 
     expect(session_keys.size).to eq(1)
-    expect(get_ttl(session_keys.first)).to eq expiration
+    expect(get_ttl(session_keys.first)).to be_within(5).of(expiration)
   end
 
   def get_session_keys