[lldb] [llgs] Support "t" vCont action

Implement support for the "t" action that is used to stop a thread.
Normally this action is used only in non-stop mode.  However, there's
no technical reason why it couldn't be also used in all-stop mode,
e.g. to express "resume all threads except ..." (`t:...;c`).

While at it, add a more complete test for vCont correctly resuming
a subset of program's threads.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D126983