refactor Test: add markOnly() for encapsulation (PR mochajs#4249)

* add markOnly instance method to test class

* add test cases for markOnly method

* use markOnly method of test class instead of accessing parent properties method

* refactor cases for test markOnly

* refactor test class unit test markOnly to exhaustively satisfy

Ref: mochajs#3689

lib/interfaces/common.js

@@ -165,7 +165,7 @@ module.exports = function(suites, context, mocha) {
        * @returns {*}
        */
       only: function(mocha, test) {
-        test.parent.appendOnlyTest(test);
+        test.markOnly();
         return test;
       },
 

lib/test.js

@@ -48,6 +48,15 @@ Test.prototype.retriedTest = function(n) {
   this._retriedTest = n;
 };
 
+/**
+ * Add test to the list of tests marked `only`.
+ *
+ * @private
+ */
+Test.prototype.markOnly = function() {
+  this.parent.appendOnlyTest(this);
+};
+
 Test.prototype.clone = function() {
   var test = new Test(this.title, this.fn);
   test.timeout(this.timeout());

test/unit/test.spec.js

@@ -2,6 +2,7 @@
 
 var mocha = require('../../lib/mocha');
 var Test = mocha.Test;
+var sinon = require('sinon');
 
 describe('Test', function() {
   describe('.clone()', function() {
@@ -83,4 +84,29 @@ describe('Test', function() {
       expect(this._test.isPending(), 'to be', true);
     });
   });
+
+  describe('.markOnly()', function() {
+    var sandbox;
+
+    beforeEach(function() {
+      sandbox = sinon.createSandbox();
+    });
+
+    afterEach(function() {
+      sandbox.restore();
+    });
+
+    it('should call appendOnlyTest on parent', function() {
+      var test = new Test('foo');
+      var spy = sandbox.spy();
+      test.parent = {
+        appendOnlyTest: spy
+      };
+      test.markOnly();
+
+      expect(spy, 'to have a call exhaustively satisfying', [test]).and(
+        'was called once'
+      );
+    });
+  });
 });