test MB-26907 backfill expired value (couchstore) intermittent cv failure

Original failure

Intermittent failure of the following test:

Running [0086/0088]: test MB-26907 backfill expired value (couchstore)

From the test suite:

52/165 Test #44: ep_testsuite_dcp.full_eviction.comp_active

Failure is:

Exceeded maximum wait time of 60000000us waiting for stat 'vb_active_expired' to be 5 (last value:10) - aborting.

See http://cv.jenkins.couchbase.com/job/kv_engine-ASan-UBSan-master/3450/

This was due to running both with and without EnableExpiryOutput within the same test. The fix for this was to separate into two tests.

Follow up failure

test MB-26907 backfill expired value - ExpiryOutput Enabled (couchstore) ...

Exceeded maximum wait time of 60000000us waiting for stat 'vb_active_expired' to be 5 (last value:4) - aborting.

You can see that this is running the new test as ExpiryOutput is clearly separated, which is the outcome from the original failure.
From this, we can now see the stat is one less than what it expects, which appears to be the same case as MB-32083.
It also appears that this issue with vb_active_expired only occurs on the threadSanitizer.