Set flag well before testing its effect

This test fails intermittently, behaving as if the feature flag isn't
enabled even though the API reports that it is.

The feature flag was set immediately before the page was loaded to
check the effect of the feature flag, so this could be a race
condition in which the API acknowledges the flag as enabled, but Rails
generates the page based on stale settings.

This changes the order of steps so that the feature flag is set as
early as possible (after the project is created), yo allow time for
possible stale flag values to expire.

qa/qa/specs/features/browser_ui/3_create/merge_request/view_merge_request_merge_ref_diff_spec.rb

@@ -3,8 +3,15 @@
 module QA
   RSpec.describe 'Create', :requires_admin do
     describe 'View merge request merge-ref diff' do
+      let(:project) do
+        Resource::Project.fabricate_via_api! do |project|
+          project.name = 'merge-ref-diff'
+        end
+      end
+
       let(:merge_request) do
         Resource::MergeRequest.fabricate_via_api! do |merge_request|
+          merge_request.project = project
           merge_request.title = 'This is a merge request'
           merge_request.description = '... for viewing merge-ref and merge-base diffs'
           merge_request.file_content = 'This exists on the source branch only'
@@ -13,16 +20,14 @@ module QA
 
       let(:new_file_name) { "added_file-#{SecureRandom.hex(8)}.txt" }
 
+      context 'when the feature flag default_merge_ref_for_diffs is enabled' do
         before do
+          Runtime::Feature.enable('default_merge_ref_for_diffs', project: project)
+
           commit_to_branch(merge_request.target_branch, new_file_name)
           commit_to_branch(merge_request.source_branch, new_file_name)
 
           Flow::Login.sign_in
-      end
-
-      context 'when the feature flag default_merge_ref_for_diffs is enabled' do
-        before do
-          Runtime::Feature.enable('default_merge_ref_for_diffs', project: merge_request.project)
 
           merge_request.visit!
         end
@@ -42,7 +47,12 @@ module QA
 
       context 'when the feature flag default_merge_ref_for_diffs is disabled' do
         before do
-          Runtime::Feature.disable('default_merge_ref_for_diffs', project: merge_request.project)
+          Runtime::Feature.disable('default_merge_ref_for_diffs', project: project)
+
+          commit_to_branch(merge_request.target_branch, new_file_name)
+          commit_to_branch(merge_request.source_branch, new_file_name)
+
+          Flow::Login.sign_in
 
           merge_request.visit!
         end