The WeakRefs proposal advanced to Stage 4 but cleanupSome moved back to a Stage 2 status.
The tests should remain in Test262 for now but this feature flag should help users to
filter tests out.
Verify that every test file which references a harness file using the
"includes" directive also contains at least one reference to a value
defined in the harness file.
To support this check, extend each harness file with a list of values
which it defines.