Disable ParserSync test for now, it takes waaaaay too long.

Running this in x64.release mode on a powerful HP620 takes 4 seconds,
this is at least 2 orders of magnitude too slow and leads to tons of
false positives on our build bots due to timeouts. As it is, the
cost-benefit ratio is far too low.

The whole approach needs to be changed: Instead of trying to exhaust
some search space in unit tests, this should be turned into a fuzzing
test where only a small but random number of things are tested. The
exhaustive approach can be done separately, but definitely not in the
unit tests.

BUG=v8:3707

Review URL: https://codereview.chromium.org/762743002

Cr-Commit-Position: refs/heads/master@{#25510}
Mark slow tests as slow.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/755283004

Cr-Commit-Position: refs/heads/master@{#25511}