This tests the combination of the prof_recent and thread_name features.
Verified that it catches the issue being fixed in this PR.
Also explicitly set thread name in test/unit/prof_recent. This fixes the name
testing when no default thread name is set (e.g. FreeBSD).
Add mtx (mutex) to test infrastructure, in order to avoid bootstrapping
complications that would result from directly using malloc_mutex.
Rename test infrastructure's thread abstraction from je_thread to thd.
Fix some header ordering issues.