The pam-auth-plugin has a Test button. All this does is to check if /etc/shadow is readable.

This is a useless test. The plugin does not depend on being able to read /etc/shadow. Furthermore, on a Linux system the shadow file by design can only be read (or written) by a very small set of programs. It is even dangerous to suggest that the jenkins user needs to have read access to that file.

I suggest to completely get rid of the test button and the doTest function.