Add missing feature category to merge request widget tests

What does this MR do and why?

Add the missing feature category for a test file and removes the test file from the exceptions list.

Screenshots or screen recordings

Running these tests: spec/serializers/merge_request_widget_entity_spec.rb Before:

After this change:

Before	After
Saw this warning when running the test "Missing metadata feature_category:"	no such warning when running the test

How to set up and validate locally

1. Ran the below command bundle exec rspec spec/serializers/merge_request_widget_entity_spec.rb
2. Montiored the test results, the screen shot above shows before and after.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.