Compression test 400 error if ajax url contains a query string

[tofandel]

If a filter for a translation plugin (eg: wpml) modifies the ajax url to contain a query string like admin-ajax.php?lang=en, then the compression test done in wp-admin/includes/template.php causes a 400 error because it contains multiple search params (?lang=en?action=wp-compression-test) in the url

The admin_url function is a function that is filtered, as such other plugins can add query string to it like ?lang=en when this is the case the request for the compression test is an invalid request that triggers error 400 because it contains multiple '?' in the url

This pull request changes the handling to handle this case

[westonruter]

This should be made obsolete by #57548

[westonruter]

I'll milestone this for visibility, but the relevant code will likely (hopefully) be deleted entirely in WP 7.2.