tests: Add some trivial tests for g_thread_init()

It’s deprecated, but the big red 0/0/0 line for the `glib/gthread`
directory in the lcov output for GLib is driving me nuts.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gthread/meson.build

@@ -36,3 +36,7 @@ pkg.generate(libgthread,
 
 libgthread_dep = declare_dependency(link_with : libgthread)
 meson.override_dependency('gthread-2.0', libgthread_dep)
+
+if build_tests
+  subdir('tests')
+endif