Stub gitlab-experiment config.cache in before each

- This shifts the stubbing behavior to all specs instead of a few.

config/initializers/gitlab_experiment.rb

 # frozen_string_literal: true
 
 Gitlab::Experiment.configure do |config|
-  config.define_singleton_method(:_default_cache) do
-    Gitlab::Experiment::Cache::RedisHashStore.new(
+  config.base_class = 'ApplicationExperiment'
+  config.cache = Gitlab::Experiment::Cache::RedisHashStore.new(
     pool: ->(&block) { Gitlab::Redis::SharedState.with { |redis| block.call(redis) } }
   )
-  end
-
-  config.base_class = 'ApplicationExperiment'
-  config.cache = config._default_cache
 end

spec/experiments/application_experiment_spec.rb

@@ -191,15 +191,15 @@ RSpec.describe ApplicationExperiment, :experiment do
   end
 
   context "when caching" do
-    let(:cache) { Gitlab::Experiment::Configuration._default_cache } # as defined in the initializer
+    let(:cache) { Gitlab::Experiment::Configuration.cache }
 
     before do
+      allow(Gitlab::Experiment::Configuration).to receive(:cache).and_call_original
+
       cache.clear(key: subject.name)
 
       subject.use { } # setup the control
       subject.try { } # setup the candidate
-
-      allow(Gitlab::Experiment::Configuration).to receive(:cache).and_return(cache)
     end
 
     it "caches the variant determined by the variant resolver" do

spec/support/gitlab_experiment.rb

@@ -12,5 +12,9 @@ class ApplicationExperiment < Gitlab::Experiment # rubocop:disable Gitlab/Namesp
   end
 end
 
-# Disable all caching for experiments in tests.
-Gitlab::Experiment::Configuration.cache = nil
+RSpec.configure do |config|
+  # Disable all caching for experiments in tests.
+  config.before do
+    allow(Gitlab::Experiment::Configuration).to receive(:cache).and_return(nil)
+  end
+end