[Test Code]: Run basic backfill unit test for persistent bucket as well

[Test Code]: Run basic backfill unit test for persistent bucket as well

The test code was trying to run flush vbucket for persistent buckets
while a flusher task was already running and hence resulting in a
temp fail of flusher.

This has been fixed by not running the flusher by explicit call.
Rather the test waits for the flusher to complete.

Change-Id: Ic77cdb518aae388baa24187ee2cf588c6bb1a609
Reviewed-on: http://review.couchbase.org/77340
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>