LU-10488 tests: fix sub-test return value issue in sanity-dom.sh

LU-10488 tests: fix sub-test return value issue in sanity-dom.sh

This patch fixes test_sanity() and test_sanityn() in sanity-dom.sh
to return the actual exit values of sanity.sh and sanityn.sh.

For bash, variable assignments preceding commands affect only that
command. So, we can just change sh to bash and do not need save
and restore the value of ONLY.

Test-Parameters: trivial

Change-Id: I1edb1022f856552cb19cb6bd713aa9b6fce37b73
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/30842
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>