Bug 1657535 - Rename runTest18 to runTest19 to run runTest18 properly in printpreview_helper.xhtml. r=emilio

There are two runTest18 functions. :)

Differential Revision: https://phabricator.services.mozilla.com/D86134

layout/base/tests/chrome/printpreview_helper.xhtml

@@ -521,7 +521,7 @@ async function runTest18() {
   requestAnimationFrame(() => setTimeout(runTest19));
 }
 
-async function runTest18() {
+async function runTest19() {
   await compareFiles("color_adjust.html", "color_adjust_ref.html", {
     test: {
       settings: {