meson: port installed tests and reftests

I couldn't get all reftests to work reliably, so the tests failing
on either CI or on my machine are skipped for now.

Installed tests are disabled by default and can be enabled with "-Dinstalled_tests=true"

testsuite/gtk/gtk.test.in

@@ -0,0 +1,4 @@
+[Test]
+Type=session
+Output=TAP
+Exec=env G_ENABLE_DIAGNOSTIC=0 @testexecdir@/@test@ --tap