This makes test outcomes immutable. This has two benefits:
1) It's more robust. Using the |= operator by mistake
wouldn't lead to unwanted mutation.
2) It's faster as now the set reference can be copied again
when copying test cases. Test case copying happens an order
of magnitude more often than rules do apply.

BUG=v8:5238

Review-Url: https://codereview.chromium.org/2212333002
Cr-Commit-Position: refs/heads/master@{#38373}