Make MTLS scenario run single threaded

What does this MR do and why?

In !990 (merged) we made this scenario run in parallel. However we've had a number of flaky failures in specs related to this scenario over the past year or so. The errors suggest that there may be some issue with the SSL certs not being correctly recongized so in the interest of ruling out any oddities that could possibly be caused by the parallel nature of this scenario setup, let's revert this back to a single threaded implementation.

As the scenario only runs a minimal number of tests compared to other jobs, it shouldn't extend the runtime of any suite that executes this test at this moment in time.

Sample flakes:

• [Test] api/12_systems/gitaly/gitaly_mtls_spec.r... (gitlab#431474 - closed)
• Failure in api/12_systems/gitaly/gitaly_mtls_sp... (gitlab#436555 - closed)
• https://gitlab.com/gitlab-org/gitlab/-/issues/472746+
• https://gitlab.com/gitlab-org/gitlab/-/issues/502246+

We have no known reproducer for any of the above failures making it difficult to determine if this will indeed address the failures but I believe it's worth attempting in any case, given the lack of progress we've managed to make otherwise in rectifying these failures.

How to set up and validate locally

$ bundle exec exe/gitlab-qa Test::Integration::MTLS EE

MR acceptance checklist

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