ci: wire up Meson builds

Wire up CI builds for both GitLab and GitHub that use the Meson build
system.

While the setup is mostly trivial, one gotcha is the test output
directory used to be in "t/", but now it is contained in the build
directory. To unify the logic across Makefile- and Meson-based builds we
explicitly set up the `TEST_OUTPUT_DIRECTORY` variable so that it is the
same for both build systems.

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

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