Bug 1388365 - Return value from assert_success. r=whimboo

Return the contents of value, if it exists, after asserting the response
was successful.

assert_success can be passed a value which will assert that the response's
value matches an expected value.  If you want to compare a subsection
of the response it may be convenient to for the value to be returned
after it has been asserted that the response was a success.

MozReview-Commit-ID: 1dyzQIazYEN

--HG--
extra : rebase_source : f628f1fa144ccfd026b21df1b4bc2e748b08413c

testing/web-platform/tests/webdriver/tests/support/asserts.py

@@ -80,6 +80,7 @@ def assert_success(response, value=None):
     assert response.status == 200
     if value is not None:
         assert response.body["value"] == value
+    return response.body.get("value")
 
 def assert_dialog_handled(session, expected_text):
     result = session.transport.send("GET",