LU-8333 test: Add replay-dual 21b to ALWAYS_EXCEPT

LU-8333 test: Add replay-dual 21b to ALWAYS_EXCEPT

Replay-dual's subtest test_21b has been failing more than half the time
(since June 2016) with a COS check failure.  Finding a fix for this
is taking longer than expected.  We would like to except this subtest
until a better COS check test can be developed.  This change only
affects the replay-dual.sh test set script.

Test-Parameters: trivial testlist=replay-dual

Change-Id: I575342ae5b6633efc198383b3351b4d9cfc1fbf0
Signed-off-by: Jim Casper <jamesx.casper@intel.com>
Reviewed-on: https://review.whamcloud.com/26631
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>