According to the coverage bot, there's some lack of test coverage for
corner cases of Math.imul(). Add the missing test coverage and also
add some coverage for the generally interesting cases.

Bug: v8:8015
Change-Id: I2a917283b4777510fb5db421a039ff0de9b2a25f
Reviewed-on: https://chromium-review.googlesource.com/1235577Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56077}