Add multi-threaded tests to the multi-threaded parallelism group

[cbezault]

In #1776 I added the standard library header units test to a parallelism group in order to mitigate #1617. We should audit our other tests and add any multi-threaded ones to this group as well.

[CaseyCarter]

This issue was created to mitigate #1617, which has since been closed do to no longer recurring. I think there's no longer any reason to make this change.