Fixed failing specs

Re-added feature flag to specs

spec/frontend/diffs/components/diff_file_spec.js

@@ -215,7 +215,7 @@ describe('DiffFile', () => {
     it('calls handleLoadCollapsedDiff if collapsed changed & file has no lines', done => {
       jest.spyOn(vm, 'handleLoadCollapsedDiff').mockImplementation(() => {});
 
-      vm.file.highlighted_diff_lines = undefined;
+      vm.file.highlighted_diff_lines = [];
       vm.file.parallel_diff_lines = [];
       vm.isCollapsed = true;
 
@@ -242,8 +242,8 @@ describe('DiffFile', () => {
 
       jest.spyOn(vm, 'handleLoadCollapsedDiff').mockImplementation(() => {});
 
-      vm.file.highlighted_diff_lines = undefined;
-      vm.file.parallel_diff_lines = [];
+      vm.file.highlighted_diff_lines = [];
+      vm.file.parallel_diff_lines = undefined;
       vm.isCollapsed = true;
 
       vm.$nextTick()

spec/spec_helper.rb

@@ -202,7 +202,7 @@ RSpec.configure do |config|
 
       # The following can be removed once we are confident the
       # unified diff lines works as expected
-      # stub_feature_flags(unified_diff_lines: false)
+      stub_feature_flags(unified_diff_lines: false)
 
       # Merge request widget GraphQL requests are disabled in the tests
       # for now whilst we migrate as much as we can over the GraphQL