Stub `ForkedStorageCheck.storage_available?` by default in all specs

Add `:broken_storage` metadata to examples to disable this behavior only
when necessary.

spec/lib/gitlab/git/storage/forked_storage_check_spec.rb

 require 'spec_helper'
-describe Gitlab::Git::Storage::ForkedStorageCheck, skip_database_cleaner: true do
+describe Gitlab::Git::Storage::ForkedStorageCheck, broken_storage: true, skip_database_cleaner: true do
   let(:existing_path) do
     existing_path = TestEnv.repos_path
     FileUtils.mkdir_p(existing_path)

spec/spec_helper.rb

@@ -105,6 +105,18 @@ RSpec.configure do |config|
     reset_delivered_emails!
   end
+  # Stub the `ForkedStorageCheck.storage_available?` method unless
+  # `:broken_storage` metadata is defined
+  #
+  # This check can be slow and is unnecessary in a test environment where we
+  # know the storage is available, because we create it at runtime
+  config.before(:example) do |example|
+    unless example.metadata[:broken_storage]
+      allow(Gitlab::Git::Storage::ForkedStorageCheck)
+        .to receive(:storage_available?).and_return(true)
+    end
+  end
+
   config.around(:each, :use_clean_rails_memory_store_caching) do |example|
     caching_store = Rails.cache
     Rails.cache = ActiveSupport::Cache::MemoryStore.new