adding a "retry" event for a test that fails but can be retried; closes

mochajs#2529 (mochajs#3408)

Continuation of mochajs#2926.  

Addresses issue mochajs#2529 by allowing reporters to take action on tests that have failed but can be retried

lib/runner.js

@@ -579,6 +579,8 @@ Runner.prototype.runTests = function(suite, fn) {
             clonedTest.currentRetry(retry + 1);
             tests.unshift(clonedTest);
 
+            self.emit('retry', test, err);
+
             // Early return + hook trigger so that it doesn't
             // increment the count wrong
             return self.hookUp('afterEach', next);

test/unit/runner.spec.js

@@ -393,6 +393,36 @@ describe('Runner', function() {
     });
   });
 
+  describe('.run(fn)', function() {
+    it('should emit "retry" when a retryable test fails', function(done) {
+      var retries = 2;
+      var retryableFails = 0;
+      var err = new Error('bear error');
+
+      var test = new Test('im a test about bears', function() {
+        if (retryableFails < retries) {
+          throw err;
+        }
+      });
+
+      suite.retries(retries);
+      suite.addTest(test);
+
+      runner.on('retry', function(testClone, testErr) {
+        retryableFails += 1;
+        expect(testClone.title, 'to be', test.title);
+        expect(testErr, 'to be', err);
+      });
+
+      runner.run(function(failures) {
+        expect(failures, 'to be', 0);
+        expect(retryableFails, 'to be', retries);
+
+        done();
+      });
+    });
+  });
+
   describe('allowUncaught', function() {
     it('should allow unhandled errors to propagate through', function(done) {
       var newRunner = new Runner(suite);