Maintaining an AST class just for testing constant exressions does not
seem justified. This CL changes constant expressions in mjsunit tests
to be represented with bytes, like regular expressions.

Change-Id: If5ec5f4d863176952442b1a7e2fec8a61e385971
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3714237Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81266}