Deprecate gtester

Since the TAP output support landed in g_test_*(), gtester has been de-facto deprecated, with an eventual plan to formally deprecate it (now) and remove it (when we next break API).

That time has come.

Let’s deprecate gtester in favour of using the tools provided by people’s build systems (Meson, autotools’ parallel testing harness, etc.) with g_test_*() feeding TAP results into them.