Workflows: Run PHP unit tests against previous WP version

[ockham]

What problem does this address?

Gutenberg is supposed to work on the current WordPress version, and on the previous one.

While we have solid test coverage for the current WP version, we don't currently have any coverage for the previous one. This sometimes results in failures when people attempt to run GB on the previous version, as seen e.g. #46811.

What is your proposed solution?

Per this discussion, PHP unit tests should catch the bulk of issues like that (while not adding as much overhead as JS unit tests, or e2e tests).

Since #46510 moves PHP unit tests from unit-tests.yml (which they share with JS unit tests) into their own workflow file, let's wait for it to land so that we can then consider adding currentWordPress - 1 to the PHP unit test matrix.

cc/ @noahtallen @anton-vlasenko