Fix flaky restrict_gitlab_schema_spec.rb

This test was failing because `Feature.enabled?(:redis_hll_tracking)`
was called in the migration and expecting the `feature_names` database
table to be accessed. However, it wasn't accessed for two reasons:

1. In tests we stub out feature flag access and use an in-memory
cache. However, even though the test had `stub_feature_flags: false`
set in the metadata, a stale `Feature` was still memoized and had to be
cleared out.

2. With the feature flag stubs disabled, this enabled the Redis cache
for feature flags. We need to clear this out between tests to ensure
the database is hit.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/416143

spec/lib/gitlab/database/migration_helpers/restrict_gitlab_schema_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 RSpec.describe Gitlab::Database::MigrationHelpers::RestrictGitlabSchema, query_analyzers: false,
-  stub_feature_flags: false, feature_category: :cell do
+  stub_feature_flags: false, use_clean_rails_redis_caching: true, feature_category: :cell do
   let(:schema_class) { Class.new(Gitlab::Database::Migration[1.0]).include(described_class) }
   # We keep only the GitlabSchemasValidateConnection analyzer running

spec/support/helpers/stub_feature_flags.rb

@@ -22,6 +22,7 @@ def stub_all_feature_flags
   def unstub_all_feature_flags
     Feature.stub = false
+    Feature.reset_flipper
   end
   # Stub Feature flags with `flag_name: true/false`