LU-3195 tests: Interop 2.3.0<->2.3.63 sanity-quota test_6

LU-3195 tests: Interop 2.3.0<->2.3.63 sanity-quota test_6

sanity-quota test_6 is picking up watchdog messages from
prevous tests. The fix is clearing dmesg at the beginning
of test_6. This also allowed us to remove some of strings
that the awk command is looking for, i.e. the subtest name.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I39b93916818b64560e82aa01d839d49c994693ce
Reviewed-on: http://review.whamcloud.com/6313
Tested-by: Hudson
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>