Tighten gdb.threads/no-unwaited-for-left.exp regexps

Tighten gdb.threads/no-unwaited-for-left.exp regexps

A WIP version of a patch
(https://sourceware.org/pipermail/gdb-patches/2022-June/190202.html)
resulted in a bug that went unnoticed by the testuite, like so:

 (gdb) PASS: gdb.threads/no-unwaited-for-left.exp: enable scheduler-locking, for main thread
 continue
 Continuing.
 [New Thread 1251861.1251861]
 No unwaited-for children left.
 (gdb) PASS: gdb.threads/no-unwaited-for-left.exp: continue stops when the main thread exits
 info threads
   Id   Target Id                                Frame
   3    Thread 1251861.1251863 "no-unwaited-for" __pthread_clockjoin_ex (threadid=140737351558976, thread_return=0x0, clockid=<optimized out>, abstime=<optimized out>, block=<optimized out>) at pthread_join_common.c:145
   4    Thread 1251861.1251861 "no-unwaited-for" <unavailable> in ?? ()

 The current thread <Thread ID 1> has terminated.  See `help thread'.
 (gdb) PASS: gdb.threads/no-unwaited-for-left.exp: only thread 3 left, main thread terminated

Somehow, above, GDB re-added the zombie leader back before printing
"No unwaited-for children left.".  The "only thread 3 left, main
thread terminated" test should have caught this, but didn't.  That is
because the test's regexp has a ".*" after the part that matches
thread 3.  This commit tightens that regexp to catch such a bug.  It
also tightens the "only main thread left, thread 2 terminated" test's
regexp in the same way.

Change-Id: I8744f327a0aa0e2669d1ddda88247e99b91cefff