Use get_user_by() in is_user_spammy()

[ocean90]

Comes up after reading #23190.

Since is_user_spammy expects a user name, we should replace get_user_id_from_string( $username ) with get_user_by( 'login', $username ). Same for the id.

I agree. We could then deprecate get_user_id_from_string(), since it's the only instance in core.

Looks good. Let's add some simple unit tests.

Does this test look sufficient?

also update function inline docs to reflect change

In 1216/tests:

Test that is_user_spammy() properly recognizes a numeric username. props bananastalktome. see #23192. see #23190.

In 23431:

Replace get_user_id_from_string() with get_user_by(). props ocean90, bananastalktome. fixes #23192. see #23190.

In 23432:

Use wp_get_current_user(). props nacin. see #23192.