Bug 34893: Unit tests for C4::Auth::checkpw

Bug 34893: Unit tests for C4::Auth::checkpw

This patch introduces some tests on the current (and new) behavior for
the `checkpw` function.

I needed it to better understand if an edge case was actually possible
(it wasn't).

Found a really minor annoyance for the internal check with expired
password not returning the $patron object for consistency with the other
use cases.

I think this method deserves (at least) changing the return value to a
sane data structure. But that's not target for backporting to stable
releases. So a separate bug.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>