selftests: tpm: Use /bin/sh instead of /bin/bash

It's better to use /bin/sh instead of /bin/bash in order to run the tests
in the BusyBox shell.

Fixes: 6ea3dfe1

 ("selftests: add TPM 2.0 tests")
Cc: stable@vger.kernel.org
Cc: linux-integrity@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>