girepository: Add a couple of very simple unit tests

For the moment, this is enough to roughly verify that the port to
`GTypeInstance` has not massively broken things. It’s not anywhere near
sufficient to qualify as a proper test suite though.

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

Helps: #3155

girepository/meson.build

@@ -252,3 +252,7 @@ if enable_gir
     extra_args: gir_args + libgirepository_gir_args,
   )
 endif
+
+if build_tests
+  subdir('tests')
+endif