[lldb] [llgs] Refactor fork/vfork tests, verify state

Refactor the fork and vfork tests to reuse the code better, avoid
unnecessary regexps and avoid unnecessary conversions between
hex-strings and integers.

Verify the server state after detaching.  In particular, verify that
the detached process' PID/TID pair is no longer valid,
and that the correct process remains running.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127290