sed: Specify shell for "nobody" user in run-ptest

sed: Specify shell for "nobody" user in run-ptest

ptest testsuite/panic-tests.sh of sed need to be run as a non-root user
so that the expected "sed: couldn't open temporary file <filename>:
Permission denied" error can be generated. After disabling default
shell for "nobody", a shell needs to be specified for running ptest.

Signed-off-by: Jiaqing Zhao <jiaqing.zhao@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>