Fallback to legacy_reader in specs

- For flaky pipeline error when ff is not stubbed correctly
- Fallback to legacy reader instead creashing the test
- Check if there are no errors added to shared

spec/lib/gitlab/import_export/project/tree_restorer_spec.rb

@@ -29,14 +29,12 @@ describe Gitlab::ImportExport::Project::TreeRestorer do
           stub_feature_flags(project_import_ndjson: ndjson_enabled)
 
           setup_import_export_config('complex')
-
-          # FIXME: Disabling `setup_reader` temporarily to always fallback to legacy_relation_reader.
-          # See https://gitlab.com/gitlab-org/gitlab/-/issues/213793#note_320801541
-          # setup_reader(reader)
+          setup_reader(reader)
 
           allow_any_instance_of(Repository).to receive(:fetch_source_branch!).and_return(true)
           allow_any_instance_of(Gitlab::Git::Repository).to receive(:branch_exists?).and_return(false)
 
+          expect(@shared).not_to receive(:error)
           expect_any_instance_of(Gitlab::Git::Repository).to receive(:create_branch).with('feature', 'DCBA')
           allow_any_instance_of(Gitlab::Git::Repository).to receive(:create_branch)
 

spec/support/import_export/common_util.rb

@@ -36,15 +36,12 @@ module ImportExport
     end
 
     def setup_reader(reader)
-      case reader
-      when :legacy_reader
-        allow_any_instance_of(Gitlab::ImportExport::JSON::LegacyReader::File).to receive(:exist?).and_return(true)
-        allow_any_instance_of(Gitlab::ImportExport::JSON::NdjsonReader).to receive(:exist?).and_return(false)
-      when :ndjson_reader
+      if reader == :ndjson_reader && Feature.enabled?(:project_import_ndjson)
         allow_any_instance_of(Gitlab::ImportExport::JSON::LegacyReader::File).to receive(:exist?).and_return(false)
         allow_any_instance_of(Gitlab::ImportExport::JSON::NdjsonReader).to receive(:exist?).and_return(true)
       else
-        raise "invalid reader #{reader}. Supported readers: :legacy_reader, :ndjson_reader"
+        allow_any_instance_of(Gitlab::ImportExport::JSON::LegacyReader::File).to receive(:exist?).and_return(true)
+        allow_any_instance_of(Gitlab::ImportExport::JSON::NdjsonReader).to receive(:exist?).and_return(false)
       end
     end