oeqa/pam: Need shadow installed for the tests

Distros may not use shadow for login manager
secondly login --help returns 1 only with shadow provided login
utility, with busybox it returns normal 0, the testcase clearly
assumes shadow being part of image, therefore add needed
checks to ensure the pre-requisites are met

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>