Ensure that validation query vars persist through redirects

[westonruter]

Summary

Given a plugin that performs a redirect, such as the contrived example which redirects any AMP page to automatically get redirected=1 added as an additional query var:

add_action(
	'template_redirect',
	function () {
		if ( function_exists( 'is_amp_endpoint' ) && is_amp_endpoint() && ! isset( $_GET['redirected'] ) ) {
			$url = wp_unslash( $_SERVER['REQUEST_URI'] );
			$url = add_query_arg( 'redirected', '1', $url );
			$url = remove_query_arg( [ 'amp_validate', 'amp_cache_bust' ], $url );
			wp_redirect( $url );
			exit;
		}
	}
);

At the moment for a site in Transitional mode, going to a non-AMP page at /about/ and clicking Validate in the admin bar results in:

The issue is that the amp_validate query var was not persisting across redirects, so this PR fixes that problem so that this is now the result when attempting to validate /about/ on a Transitional mode site:

This issue came up specifically where WPML was attempting to redirect the homepage to /en/ in this support topic: https://wordpress.org/support/topic/url-validation-failed-due-to-unexpected-json-in-amp-validation-response/#post-12637035

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[pierlon]

Verified in QA; the query vars amp_validate and amp_cache_bust are persisted when redirecting.