glib-genmarshal: Fix ref-sinking of GVariants in valist marshallers

The old (Perl) implementation of glib-genmarshal used
g_variant_ref_sink() to correctly handle floating inputs; the Python
version should do the same.

Includes a unit test.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1793

gobject/glib-genmarshal.in

@@ -360,7 +360,7 @@ IN_ARGS = {
         'signal': 'VARIANT',
         'ctype': 'gpointer',
         'getter': 'g_marshal_value_peek_variant',
-        'box': ['g_variant_ref', 'g_variant_unref'],
+        'box': ['g_variant_ref_sink', 'g_variant_unref'],
         'static-check': True,
         'takes-type': False,
     },