This ensures that tests actually abort on unimplemented opcodes instead
of just printing them as "Unimplemented Instruction". If used to
disassemble a code region though, we want to ignore unimplemented
opcodes to keep printing remaining valid instructions.

The tests were previously fixed by Deepti in
8fa509d3, but this got partly reverted
on the "Address" refactoring in
2459046c.

R=titzer@chromium.org

Change-Id: I802dda2b0f45ee77c4f9b244ed984b1c4679bac3
Reviewed-on: https://chromium-review.googlesource.com/1146649
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54726}