common/rc: fix false positives due to leaked mkfs output to stderr

common/rc: fix false positives due to leaked mkfs output to stderr

The change fixes false positives reported by a few generic tests under
circumstances, when an mkfs utility prints its output to stderr and then it
appears in produced *.out files. The change is intended to fix such issues
in the tests, which utilize _require_scratch_swapfile().

Signed-off-by: Vladimir Zapolskiy <vladimir@tuxera.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>