Makefile: detect missing Meson tests

Makefile: detect missing Meson tests

In the preceding commit, we have introduced consistency checks to Meson
to detect any discrepancies with missing or extraneous tests in its
build instructions. These checks only get executed in Meson though, so
any users of our Makefiles wouldn't be alerted of the fact that they
have to modify the Meson build instructions in case they add or remove
any tests.

Add a comparable test target to our Makefile to plug this gap.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>