Always emit =thread-exited notifications, even if silent

[Note: the testcased added by this commit depends on
https://sourceware.org/pipermail/gdb-patches/2022-June/190259.html,
otherwise GDB just crashes when detaching the core]

Currently, in MI, =thread-created are always emitted, like:

 =thread-group-started,id="i1",pid="195680"
 =thread-created,id="1",group-id="i1"
 ...

but on teardown, if the target uses exit_inferior_silent, then you'll
only see the inferior exit notification (thread-group-exited), no
notification for threads.

The core target is one of the few targets that use
exit_inferior_silent.  Here's an example session:

 -target-select core $corefile
 =thread-group-started,id="i1",pid="195680"
 =thread-created,id="1",group-id="i1"
 ...
 ^connected,frame=....
 (gdb)
 -target-detach
 =thread-group-exited,id="i1"
 ^done
 (gdb)

This imbalance of emitting =thread-created but then not =thread-exited
seems off to me.  (And, it complicates changes I want to do to
centralize emitting thread exit notifications for the CLI, which is
why I'm looking at this.)

And then, since most other targets use exit_inferior instead of
exit_inferior_silent, MI is already emitting =thread-exited
notifications when tearing down an inferior, for most targets.

This commit makes MI always emit the =thread-exited notifications,
even for exit_inferior_silent.

Afterwards, when debugging a core, MI outputs:

 (gdb)
 -target-detach
 =thread-exited,id="1",group-id="i1"    << new line
 =thread-group-exited,id="i1"
 ^done
 (gdb)

Surprisingly, there's no MI testcase debugging a core file.  This
commit adds the first.

Change-Id: I5100501a46f07b6bbad3e04d120c2562a51c93a4