This reverts commit b36c87e3.

Reason for revert: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux%20-%20noi18n%20-%20debug/42210/overview

Original change's description:
> [test] Move cctest/test-regexp to unittests
>
> ... /regexp/regexp-unittest.
>
> Bug: v8:12781
> Change-Id: I3c3ba4c519ff503b242c39d0e3b2350f25d7e84c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3607370
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: 王澳 <wangao.james@bytedance.com>
> Cr-Commit-Position: refs/heads/main@{#80217}

Bug: v8:12781
Change-Id: I0c5fae4b156df0a305b83acfa639bb0ff6eeb87f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3610626
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80229}