These are no longer tied to instruction-selector, so move them out into
their own unittests. We can then remove the *ForTesting methods.

Bug: v8:10696
Change-Id: I387cf38290d9602b011ee1d13ee5285ac660f208
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2326951Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69149}