test(smokehouse): fix flakiness of dom-size expectation (#1881)

adds support for `RegExp` tests in smokehouse expected values

lighthouse-cli/test/smokehouse/pwa-expectations.js

@@ -83,7 +83,7 @@ module.exports = [
         extendedInfo: {
           value: {
             1: {value: '20'},
-            2: {snippet: 'Element with most children:\nul#versionlist.canaryisdev'}
+            2: {snippet: /ul.versionlist/}
           }
         }
       },

lighthouse-cli/test/smokehouse/smokehouse.js

@@ -111,9 +111,13 @@ function findDifference(path, actual, expected) {
     return null;
   }
 
+  if (typeof actual === 'string' && expected instanceof RegExp && expected.test(actual)) {
+    return null;
+  }
+
   // If they aren't both an object we can't recurse further, so this is the difference.
   if (actual === null || expected === null || typeof actual !== 'object' ||
-      typeof expected !== 'object') {
+      typeof expected !== 'object' || expected instanceof RegExp) {
     return {
       path,
       actual,