Add multi-target tests

This adds a testcase exercising multi-target features.  It spawns 6
inferiors, like this:

 inferior 1 -> native
 inferior 2 -> extended-remote 1
 inferior 3 -> core
 inferior 4 -> native
 inferior 5 -> extended-remote 2
 inferior 6 -> core

and then tests various details, including:

 - running to breakpoints

 - interrupting with Ctrl-C and "interrupt -a"

 - "next" bouncing between two breakpoints in two threads running in
   different targets.

 - since we have cores and live inferiors mixed in the same session,
   this makes sure that gdb doesn't try to remove a core dump's
   threads.

 - all-stop and non-stop modes.

This testcase caught a _lot_ of bugs in development.

gdb/testsuite/ChangeLog:
2020-01-10  Pedro Alves  <palves@redhat.com>

	* gdb.multi/multi-target.c: New file.
	* gdb.multi/multi-target.exp: New file.
	* lib/gdbserver-support.exp (gdb_target_cmd): Handle "Non-stop
	mode requested, but remote does not support non-stop".

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