Currently the disassembler tests continue on unimplemented instructions, fix
to abort instead as most bugs in the disassemblers will appear as unimplemented
instructions.

 - Localize testing disassembler method to abort on unimplemented
 - Fix failing x64 disassembler tests

Change-Id: I703cca9709c528327ec381d05a78cf3314ea4fa9
Reviewed-on: https://chromium-review.googlesource.com/907489Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51162}