gdb/testsuite: introduce is_target_non_stop helper proc

gdb/testsuite: introduce is_target_non_stop helper proc

I noticed that several tests included copy & pasted code to run the
'maint show target-non-stop' command, and then switch based on the
result.

In this commit I factor this code out into a helper proc in
lib/gdb.exp, and update all the places I could find that used this
pattern to make use of the helper proc.

There should be no change in what is tested after this commit.

Reviewed-By: Pedro Alves <pedro@palves.net>