Add rake task to run RSpec on tests that failed in merge request

What does this MR do?

Part of #325247 (closed)

Add new rake task to run all the failed RSpec tests in an MR locally:

bin/rake spec:merge_request_rspec_failure

There are a few caveats for this Rake task:

• You need to be on the same branch on your machine as the source branch of the Merge Request.
• The pipeline must have been completed.
• You may need to wait for the test report to be parsed and retry again.

This Rake task depends on the unit test reports feature, which only gets parsed when it is requested for the first time.

Screenshots (strongly suggested)

Example:

1. When the MR test report is not ready

2. When there is no rspec failure

3. When the MR has rspec failures, it runs rspec with the failed test files (simulated failures).