[parser/test] Simplify test-preparser/PreParserScopeAnalysis.

BUG=v8:5516
R=neis@chromium.org

Change-Id: Iabc8d942ffb8107c5284cc51654792a9e2872d0a
Reviewed-on: https://chromium-review.googlesource.com/439325
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43001}