meson: It's useful downstream to be able to control building tests,

as most of Gentoo users will not be running the testing phase for
their local builds.

Co-authored-by: Sam James <sam@gentoo.org>
6 jobs for make-tests-optional in 1 minute and 40 seconds (queued for 26 seconds)
latest
Status Name Job ID Coverage
  Container
passed container_fedora #13664402

00:00:28

passed container_ubuntu #13664403

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  Build
passed build_on_fedora_no_docs #13664405

00:01:03

passed build_on_fedora_with_docs #13664404

00:01:10

passed build_on_ubuntu_no_gir #13664407

00:00:43

passed build_on_ubuntu_with_gir #13664406

00:01:03