tests: pam: use absolute path to module .so

tests: pam: use absolute path to module .so

This is a valid configuration and both (a) skips the tests if it's
unbuilt/not installed and (b) makes it work even if installed outside
the system directory (like in /u/l/l/s instead of /l/s)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259