[lldb] [llgs] Add base nonstop fork/vfork tests

Extend the most of baseline fork tests to run in nonstop mode as well.
For more cases, we're just testing one example scenario to save time.
This patch does not cover tests that rely on correct exit handling,
as fixing that is addressed in a followup patch.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D128638