Fix instanceStateValues xhr test

Writing to a read-only property or trying to deleting will throw an exception
in strict mode and thus abort the execution. Make the test work in strict and
non-strict mode by surrounding the offending statements with individual
try/catch pairs.

Change-Id: I25f803b6ffd7ef721ac874f53a37b3208b5c05ab
Reviewed-by: Lars Knoll <lars.knoll@digia.com>