LU-8900 tests: add snapshot in racer tests

LU-8900 tests: add snapshot in racer tests

To check whether snapshot may cause trouble or not.

Test-Parameters: trivial envdefinitions=SLOW=yes,RACER_ENABLE_SNAPSHOT=true mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4 testlist=racer
Test-Parameters: trivial envdefinitions=SLOW=yes,RACER_ENABLE_SNAPSHOT=false mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4 testlist=racer
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I09dababec7175c95f1a21519460e75e82ef7bc2f
Reviewed-on: https://review.whamcloud.com/26199
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>