Add unit tests for Two_Factor_Provider::get_code().

[dd32]

This was split out of #518 as it's technically unrelated, and is failing for some reason in GitHub actions, but passing locally.

I can't really determine by looking at the code why it's failing though.

[dd32]

This fails on WordPress 6.0 due to https://core.trac.wordpress.org/ticket/55194

Opened #523 to run unit tests against trunk as well.

edit: I had included a check for WP < 6.1 and skipping the test, but I've thrown that out of this PR as it's not needed if we test with stable WordPress as in #524

[dd32]

In making this testable, since Two_Factor_Provider is an abstract class, I chose to simply make it a static method due to it's lack of $this-> usage. $this->get_code() is still acceptable, but now Two_Factor_Provider::get_code() also is.

[iandunn]

Looks like this is working after #524 👍🏻