Improve spec coverage for RuboCop rules

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Problem

After enabling simplecov in "fast specs" and running specs for RuboCop rules with code coverage enabled we are seeing that some 👮 rules are (partly) missing specs:

SIMPLECOV=1 bin/rspec -f p $(find spec/rubocop -name "*_spec.rb")

The list

MR	File	% covered	Lines
!97497 (merged)	rubocop/cop/gitlab/avoid_feature_get.rb	0.00 %	27
!97493 (merged)	rubocop/cop/gitlab/keys_first_and_values_first.rb	0.00 %	57
	rubocop/cop/avoid_route_redirect_leading_slash.rb	80.95 %	52
	rubocop/cop/gitlab/mark_used_feature_flags.rb	88.72 %	282
	rubocop/cop/performance/ar_count_each.rb	90.00 %	45
	rubocop/cop/performance/ar_exists_and_present_blank.rb	92.31 %	57
	rubocop/cop/migration/update_column_in_batches.rb	95.45 %	45
	rubocop/cop/gitlab/finder_with_find_by.rb	95.83 %	54
	rubocop/cop/migration/add_concurrent_foreign_key.rb	95.83 %	53
!143663 (merged)	rubocop/cop/migration/add_limit_to_text_columns.rb	97.18 %	168
	rubocop/cop/gitlab/feature_available_usage.rb	98.04 %	99

Screenshot

Line coverage for rubocop/*

Proposed solution

Bring test coverage to 100% for each for the 👮 rules above.

Implementation Guide

1. Pick a 👮 rule from the table above ☝️ .
2. Create a well-named branch 355012-rubocop-improve-spec-coverage-<Cop/Name>.
   • Hint: Prefixing a branch name with an issue IID (355012) will add required labels and references to this issue in the new merge request.
3. Inspect resulting line coverage in your browser via file:///path-to-your-gdk/coverage/index.html#_Tooling
   • You will need to Create coverage report locally via SIMPLECOV=1 bin/rspec <rubocop spec>
4. Write specs for lines which aren't covered by a spec.
5. Create a merge request and link the MR in the table above replacing <!-- Link MR --> with !<MR-IID>.
6. 🎉