selftests: do not require bash for the generated test

Nothing in this minimal script seems to require bash. We often run these
tests on embedded devices where the only shell available is the busybox
ash. Use sh instead.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Cc: stable@vger.kernel.org
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>