Site Health: Add the test for plugin and theme auto-updates to `WP_Site_Health::get_tests()`.

This ensures that the test actually runs.

Follow-up to [48548].

See #50662.
Built from https://develop.svn.wordpress.org/trunk@48558


git-svn-id: https://core.svn.wordpress.org/trunk@48320 1a063a9b-81f0-0310-95a4-ce76da25c4cd