Move them from post-mvp to relaxed-simd flag.

We will have a follow up change to move all the relaxed-simd tests into
their own file.

Bug: v8:11583
Change-Id: Iea9809a309bac428c856e5d0bd024fe0070d5921
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773898Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73524}