LU-13759 test: make sanityn test_20 repeatable

LU-13759 test: make sanityn test_20 repeatable

- make sanityn.sh test_20 able to work with ONLY_REPEAT
  parameter by using $tdir and $tfile variable which are
  cleaned up by test framework and test can be repeated
- change sanity-dom.sh way to define sanity.sh and sanityn.sh
  parameters to allow selected tests run by using SANITY_ONLY,
  SANITYN_ONLY to choose specific tests. It supports also
  SANITY_REPEAT/SANITYN_REPEAT to repeat those tests.

Test-Parameters: trivial testlist=sanity-dom env=SANITY_ONLY=36,SANITYN_ONLY=20,SANITYN_REPEAT=50
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I314703006a8f53092daf1359f4c4694c704354d2
Reviewed-on: https://review.whamcloud.com/39540
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>