REST API: Add test for creating a comment with an invalid post ID.

#38816 fixed creating a comment with an invalid post ID (this should not be
allowed), but we need a test for this.

Fixes #38991.

Built from https://develop.svn.wordpress.org/trunk@39375


git-svn-id: https://core.svn.wordpress.org/trunk@39315 1a063a9b-81f0-0310-95a4-ce76da25c4cd