More specific test for a bad callback in `WP_Customize_Partial` test.

The 'render_callback' passed to `WP_Customize_Partial` must either echo or
return a result - not both. When it's detected that the callback echoes and
returns a value, the return value takes precedence. This is now reflected in
the corresponding unit test.

Introduced in [36586].

See #27355. See #36016.
Built from https://develop.svn.wordpress.org/trunk@36784


git-svn-id: https://core.svn.wordpress.org/trunk@36751 1a063a9b-81f0-0310-95a4-ce76da25c4cd