Restore fabrication of fork MR via browser

Resource::MergeRequestFromFork can't fabricate via the API, so it was
fabricating a regular merge request instead.

This test _should_ fabricate via the browser anyway, because it is the
main test of the fork feature.

qa/qa/resource/merge_request_from_fork.rb

@@ -28,6 +28,10 @@ module QA
         Page::Project::Show.perform(&:new_merge_request)
         Page::MergeRequest::New.perform(&:create_merge_request)
       end
+
+      def fabricate_via_api!
+        raise NotImplementedError
+      end
     end
   end
 end

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

@@ -2,21 +2,23 @@
 
 module QA
   RSpec.describe 'Create' do
-    describe 'Merge request creation from fork' do
-      let(:merge_request) do
-        Resource::MergeRequestFromFork.fabricate_via_api! do |merge_request|
+    describe 'Merge request creation from fork', :smoke do
+      let!(:merge_request) do
+        Resource::MergeRequestFromFork.fabricate_via_browser_ui! do |merge_request|
           merge_request.fork_branch = 'feature-branch'
         end
       end
 
       it 'can merge feature branch fork to mainline', testcase: 'https://gitlab.com/gitlab-org/quality/testcases/-/issues/928' do
-        Flow::Login.sign_in
-
+        Flow::Login.while_signed_in do
           merge_request.visit!
 
-        Page::MergeRequest::Show.perform(&:merge!)
+          Page::MergeRequest::Show.perform do |merge_request|
+            merge_request.merge!
 
-        expect(page).to have_content('The changes were merged')
+            expect(merge_request).to have_content('The changes were merged')
+          end
+        end
       end
     end
   end