Makefile: add support for 'make testonly'

Makefile: add support for 'make testonly'

That skips any attempt to recompile before running the tests.
Some times that's useful for debugging and we'll
use it to split the build and test stages in autobuild and gitlab-ci
later.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14628

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 1e4714940211b10ae6574770f15b7c6ed95f5f59)
(cherry picked from commit 9fed2749c039164794faadef71aa83cfd360d130)