Bug 1656216 - Improve SIMD test cases. r=jseward

This fleshes out the test cases to cover some corner cases that were
left uncovered before.  No errors were found.

Differential Revision: https://phabricator.services.mozilla.com/D85981

UltraBlame original commit: 0cabb34381855218b4032df87f53117df0aefecd

Author: marco-c