MB-21725: test_exp_persisted_set_del check for greater or equal 1

MB-21725: test_exp_persisted_set_del check for greater or equal 1

The test stores key twice and waits for ep_total_persisted to equal 1.
The test fails because the wait_for stat function timesout, the
total persisted is 2.

If the flusher ran quickly we may actually have a value of 2.

Change-Id: I18d8e50e5a5bcc4839830adb30e104bb6796daa0
Reviewed-on: http://review.couchbase.org/70091
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>