Re-enables and fixes msan test failures due to string padding being
cleared only selectively in tests. This change instead makes sure it
always happens in TestIsolate.

Bug: v8:7746
Change-Id: I259b43ad25cb7af18bf16d29effb15772c981a67
Reviewed-on: https://chromium-review.googlesource.com/1051647Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53095}