This CL also reorganizes the Strings test suite

Bug: v8:7340
Change-Id: I54d4d76a16c362e38ebfc9719ac8cb1a490ef3cc
Reviewed-on: https://chromium-review.googlesource.com/941122Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51631}