Add job for custom tests

Adds a manual job that lets you specify the scenario, release, tests, and tags, allowing you to run any tests on demand.

• QA_SCENARIO - the scenario to run (default Test::Instance::Image)
• QA_TESTS - the test(s) to run (no default, which means run all the tests in the scenario)
• QA_RSPEC_TAGS - the RSpec tags to add (no default)

Useful if you want to run a specific test a few times, or a subset of tests in a particular order.

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Update: Unfortunately, this isn’t as useful as intended because for now it’s not possible to retry the job with the custom variables

• https://gitlab.com/gitlab-org/gitlab/issues/32712
• https://gitlab.com/gitlab-org/gitlab/issues/31367