Expected outcomes optimized to serialize [PASS] as None.

Keeping expected outcomes inside output processors should be
optimized in the future. Few possible optimizations:
- separate classes for tests that are expected to PASS - done as
an example in mozilla test suite.
- cache output processors inside testcase.
- share output processors between copies of the same test - needs
some updates to the create_variant to update outproc only if
expected outcomes changed.

Bug: v8:6917
Change-Id: Ie73f1dcdf17fdfc65bce27228f818b1dd1e420c9
Reviewed-on: https://chromium-review.googlesource.com/843025
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50347}