Declare an explicit 'order' in `test_get_comments_by_user()`.

This change fixes broken builds on older versions of MySQL. See #30462.

Assertions regarding indeterminate secondary ORDER BY clauses will go in
separate test methods. See #30478.

git-svn-id: https://develop.svn.wordpress.org/trunk@30548 602fd350-edb4-49c9-b593-d223f7449a82