Tests: Skip ETag AJAX tests on TestSwarm

TestSwarm is now proxied via Cloudflare which cuts out headers relevant for
ETag tests, failing them. We're still running those tests in Karma on Chrome
& Firefox (including Firefox ESR).

Closes gh-4974

(cherry picked from commit 00c060d)

Author: mgol

test/unit/ajax.js

@@ -1557,12 +1557,25 @@ QUnit.module( "ajax", {
 		function( label, cache ) {
 			jQuery.each(
 				{
-					"If-Modified-Since": "mock.php?action=ims",
-					"Etag": "mock.php?action=etag"
+					"If-Modified-Since": {
+						url: "mock.php?action=ims",
+						qunitMethod: "test"
+					},
+					"Etag": {
+						url: "mock.php?action=etag",
+
+						// Support: TestSwarm
+						// TestSwarm is now proxied via Cloudflare which cuts out
+						// headers relevant for ETag tests, failing them. We're still
+						// running those tests in Karma on Chrome & Firefox (including
+						// Firefox ESR).
+						qunitMethod: QUnit.isSwarm ? "skip" : "test"
+					}
 				},
-				function( type, url ) {
-					url = baseURL + url + "&ts=" + ifModifiedNow++;
-					QUnit.test( "jQuery.ajax() - " + type + " support" + label, function( assert ) {
+				function( type, data ) {
+					var url = baseURL + data.url + "&ts=" + ifModifiedNow++;
+					QUnit[ data.qunitMethod ]( "jQuery.ajax() - " + type +
+							" support" + label, function( assert ) {
 						assert.expect( 4 );
 						var done = assert.async();
 						jQuery.ajax( {