This patch introduces the support::make_test_jthread utility which is
basically the same as support::make_test_thread but for std::jthread. It
allows vendors to maintain a downstream way to create threads for use
within the test suite, which is especially useful for embedded
platforms.
We create threads using std::thread in various places in the test suite.
However, the usual std::thread constructor may not work on all platforms,
e.g. on platforms where passing a stack size is required to create a thread.
This commit introduces a simple indirection that makes it easier to tweak
how threads are created inside the test suite on various platforms. Note
that tests that are purposefully calling std::thread's constructor directly
(e.g. because that is what they're testing) were not modified.
