The lookbehind tests weren’t being skipped in the “without
lookbehind” case, resulting in a redundant test.

Bug: v8:4545
Change-Id: Ia004508373ba6ec49fd9de690909467eb0a15361
Reviewed-on: https://chromium-review.googlesource.com/765468Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49339}