[lldb] [test] Use raise(SIGSTOP) instead of trap in fork tests

Replace the use of "trap" with a new "stop" command in fork tests,
that maps to `raise(SIGSTOP)`.  Since traps do not increment PC on some
architectures (notably ARM), using traps would require special logic
to increment it while testing.  Using SIGSTOP avoids the problem
and is probably more logical, given that the purpose of the "trap"s
was to simply stop the inferior at a synchronization point.  This fixes
tests on AArch64 (and possibly ARM, I'll update XFAILs when it is
confirmed by the buildbot).

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D128780