LU-3304 tests: sanity-quota test_18 watchdog triggered

LU-3304 tests: sanity-quota test_18 watchdog triggered

sanity-quota test_18 is picking up watchdog messages from
prevous tests. The fix is clearing dmesg at the beginning
of test_18. This also allowed us to remove some of strings
that the awk command looking for, i.e. the subtest name.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes \
testlist=sanity-quota

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I9a20bd2e374048f252219038df3590fef339a672
Reviewed-on: http://review.whamcloud.com/6310
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>