fix(core): measure perf for async checks (#4609)

Performance of async checks isn't measured correctly. No tests, since we
don't generally test perf timer stuff.

lib/core/base/rule.js

@@ -284,15 +284,21 @@ Rule.prototype.run = function run(context, options = {}, resolve, reject) {
     });
   });
 
-  // Defer the rule's execution to prevent "unresponsive script" warnings.
-  // See https://github.com/dequelabs/axe-core/pull/1172 for discussion and details.
-  q.defer(res => setTimeout(res, 0));
-
-  if (options.performanceTimer) {
-    this._logRulePerformance();
-  }
-
-  q.then(() => resolve(ruleResult)).catch(error => reject(error));
+  q.then(() => {
+    if (options.performanceTimer) {
+      this._logRulePerformance();
+    }
+    // Defer the rule's execution to prevent "unresponsive script" warnings.
+    // See https://github.com/dequelabs/axe-core/pull/1172 for discussion and details.
+    setTimeout(() => {
+      resolve(ruleResult);
+    }, 0);
+  }).catch(error => {
+    if (options.performanceTimer) {
+      this._logRulePerformance();
+    }
+    reject(error);
+  });
 };
 
 /**