Enable running simplecov in fast specs via `SIMPLECOV=1`

What does this MR do and why?

This MR adds simplecov functionality to "fast specs" (specs which require 'spec/fast_spec_helper') so we can see test coverage for them as well. See https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#coverage.

It also tweaks a RuboCop spec to use require 'spec/fast_spec_helper' so the command below does not fail with:

Detected fast_spec_helper is loaded first than spec_helper.
If running test files using both spec_helper and fast_spec_helper,
make sure test file with spec_helper is loaded first.
Aborting...
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

But why now?

Now, with https://docs.gitlab.com/ee/development/pipelines.html#undercover-rspec-test we encourage developers to have a code coverage of 100%. undercover wasn't available at time of writing some of the cops.

How to set up and validate locally

With simplecov enabled

$ SIMPLECOV=1 bin/rspec -f p $(find spec/rubocop -name "*_spec.rb")
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 29880
.......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 1.49 seconds (files took 2.5 seconds to load)
1031 examples, 0 failures

Randomized with seed 29880

Coverage report generated for RSpec to /home/peter/devel/gitlab/gdk-ee/gitlab/coverage. 2573 / 334560 LOC (0.77%) covered.

Coverage report generated for RSpec to /home/peter/devel/gitlab/gdk-ee/gitlab/coverage/coverage.xml
Lcov style coverage report generated for RSpec to /home/peter/devel/gitlab/gdk-ee/gitlab/coverage/lcov/gitlab.lcov

With simplecov disabled

$ bin/rspec -f p $(find spec/rubocop -name "*_spec.rb")
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 7399
.......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 2.32 seconds (files took 2.35 seconds to load)
1031 examples, 0 failures

Randomized with seed 7399

Follow up

It seems we are missing specs for some 👮 rules:

Line coverage for rubocop/*

See #355012

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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