perf: defer rules rather than checks (#1308)

This patch removes `setTimeout()` call from `Check#run()` and adds it to `Rule#run()`. The `setTimeout()` was originally added in 4657dc5 in order to prevent browsers from "freezing up" while running 1000s of synchronous checks. This had the downside of creating ~10k timers which slows down `axe.run()` considerably.

Moving the `setTimeout()` to `Rule#run()` ensures we continue to run asynchronously to prevent "unresponsive script" warnings, but we defer as little as possible. With this patch, we should see ~40 timers created rather than 10k+ since we now create one per Rule rather than one per Check.

Hat tip to @paulirish for starting this conversation in #1172.


## Reviewer checks

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- [x] Follows the commit message policy, appropriate for next version
- [x] Has documentation updated, a DU ticket, or requires no documentation change
- [x] Includes new tests, or was unnecessary
- [x] Code is reviewed for security by: @WilcoFiers

lib/core/base/check.js

@@ -90,9 +90,7 @@ Check.prototype.run = function(node, options, context, resolve, reject) {
 
 		if (!checkHelper.isAsync) {
 			checkResult.result = result;
-			setTimeout(function() {
-				resolve(checkResult);
-			}, 0);
+			resolve(checkResult);
 		}
 	} else {
 		resolve(null);

lib/core/base/rule.js

@@ -207,6 +207,10 @@ Rule.prototype.run = function(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(resolve => setTimeout(resolve, 0));
+
 	if (options.performanceTimer) {
 		axe.utils.performanceTimer.mark(markEnd);
 		axe.utils.performanceTimer.measure(