MB-27539: Fix ThreadSanitizer false positives on PersistToTest
MB-27539: Fix ThreadSanitizer false positives on PersistToTest

Tests in the PersistToTest set run with the ShutdownMode::Clean and
ShutdownMode::Unclean variants. When memcached is killed
(ShutdownMode::Unclean), ThreadSanitizer detects false race conditions
on the following tests. To fix, we skip 'Unclean' tests when run under
ThreadSanitizer.

Change-Id: I0f66151d664ba495de9ec0bc0e1a1788be44d65a
Reviewed-on: http://review.couchbase.org/87789
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine