fixes #704 - provide an 'exit' tester event to hook before runner exits

casper.test.on("exit", function() {
  console.log("exited");
})

casper.test.begin("sample", function(test) {
  test.assert(true);
  test.done();
});

will print "exited" on the last line of the output.

@@ -1524,6 +1524,7 @@ Tester.prototype.renderResults = function renderResults(exit, status, save) {
         this.saveResults(save);
     }
     if (exit === true) {
+        this.emit("exit");
         this.casper.exit(status ? ~~status : exitStatus);
     }
 };

@@ -317,6 +317,11 @@ class TestCommandOutputTest(CasperExecTestBase):
         ], failing=True)
 
     @timeout(20)
+    def test_exit_test(self):
+        script_path = os.path.join(TEST_ROOT, 'tester', 'exit.js')
+        self.assertCommandOutputContains('test ' + script_path, ['exited'])
+
+    @timeout(20)
     def test_skipped_test(self):
         script_path = os.path.join(TEST_ROOT, 'tester', 'skipped.js')
         self.assertCommandOutputContains('test ' + script_path, [