Bugfixes and mustpass update

 * Fix glReadBuffer() negative test to match latest spec language.

 * Remove said test from ES3.1 mustpass as it has been changed just
before release.

 * Remove SSO case from mustpass that has been removed earlier from the
test code.

Change-Id: Id77f7957f503121bde6b41c12cdc0141324f28bb
4 files changed