tests: Run all tests with deprecated property warnings enabled

The tests that functionally rely on G_ENABLE_DIAGNOSTIC=1 still set it
explicitly, so that they will behave as expected when run as
installed-tests or manually.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -2373,6 +2373,7 @@ endif
 
 common_test_env = [
   'G_DEBUG=gc-friendly',
+  'G_ENABLE_DIAGNOSTIC=1',
   'MALLOC_CHECK_=2',
 ]
 test_timeout = 60