[generators] Get better clusterfuzz coverage.

Duplicate mjsunit/harmony/generators.js but explicitly require
--turbo-from-bytecode such that Clusterfuzz can thoroughly test the
new generators implementation.

BUG=v8:4907

Review-Url: https://codereview.chromium.org/2002983002
Cr-Commit-Position: refs/heads/master@{#36441}