Extract stub_settings_source into spec support file

Rename `stubbing_settings_source` to `stub_settings_source` to keep
consistency across other support files.

Require this support file in spec where `stub_settings_source: true` is
used.

This avoids order dependent flaky specs like:

  bin/rspec \
    spec/commands/sidekiq_cluster/cli_spec.rb \
    spec/lib/gitlab/pagination/offset_header_builder_spec.rb

Note, both specs about are run with `fast_spec_helper` so previously
they would fail because RSpec trait `stubbing_settings_source` was not
available.

spec/commands/sidekiq_cluster/cli_spec.rb

@@ -3,9 +3,10 @@
 require 'fast_spec_helper'
 require 'rspec-parameterized'
 
+require_relative '../../support/stub_settings_source'
 require_relative '../../../sidekiq_cluster/cli'
 
-RSpec.describe Gitlab::SidekiqCluster::CLI, stubbing_settings_source: true do # rubocop:disable RSpec/FilePath
+RSpec.describe Gitlab::SidekiqCluster::CLI, stub_settings_source: true do # rubocop:disable RSpec/FilePath
   let(:cli) { described_class.new('/dev/null') }
   let(:timeout) { Gitlab::SidekiqCluster::DEFAULT_SOFT_TIMEOUT_SECONDS }
   let(:default_options) do

spec/spec_helper.rb

@@ -462,14 +462,6 @@ RSpec.configure do |config|
     $stdout = STDOUT
   end
 
-  config.around(:each, stubbing_settings_source: true) do |example|
-    original_instance = ::Settings.instance_variable_get(:@instance)
-
-    example.run
-
-    ::Settings.instance_variable_set(:@instance, original_instance)
-  end
-
   config.disable_monkey_patching!
 end
 

spec/support/stub_settings_source.rb

+# frozen_string_literal: true
+
+RSpec.configure do |config|
+  config.around(:each, stub_settings_source: true) do |example|
+    original_instance = ::Settings.instance_variable_get(:@instance)
+
+    example.run
+
+    ::Settings.instance_variable_set(:@instance, original_instance)
+  end
+end