merge_pr_48965

Fix #7169: rewrite assert_readonly to check property descriptors

This changes the implementation to only check that the property
descriptor makes the property readonly; it doesn't actually check its
behaviour. As detailed in the above issue, there's too many ways for
this test to be otherwise bogus, thus we should go for the simple
implementation.

This also fixes /css/cssom/cssimportrule.html, which previously
asserted that CSSImportRule#media was readonly, which was a
meaningless test, as even after the [PutForwards] [[Set]] call had
occured, the attribute being tested still returned the same object,
because it is defined with [SameObject].