meson: Build tests conditionally

It's useful downstream to be able to control building
tests, as there's not much use building them if we're
not going to run them.

Signed-off-by: Sam James <sam@gentoo.org>
6 jobs for conditional-tests in 1 minute and 36 seconds (queued for 22 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #13678498

00:00:27

passed container_ubuntu #13678499

00:00:34

 
  Build
passed build_on_fedora_no_docs #13678501

00:00:50

passed build_on_fedora_with_docs #13678500

00:01:00

passed build_on_ubuntu_no_gir #13678503

00:00:49

passed build_on_ubuntu_with_gir #13678502

00:00:54