selftest/gdb_backtrace: avoid printing backtrace twice with 1 thread
We call 'thread apply all bt full' in case there are interesting
things going on in other threads, but often there are no other threads
and it only serves to repeat the original trace (and very slowly, for
some reason).
The $_inferior_thread_count convenience variable is new in gdb 13.1
(2022-ish) so we init-if-undefined it to default to the old behaviour.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>