test(node-unit/mocha.spec.js): Refactor and beef up tests

Migrated `Mocha#addFiles` here. Created `Mocha#loadFiles`. Added chainable tests where applicable.
Refactored.

test/node-unit/mocha.spec.js

@@ -6,11 +6,46 @@ const utils = require('../../lib/utils');
 
 describe('Mocha', function() {
   const opts = {reporter: utils.noop}; // no output
+  const testFiles = [
+    __filename,
+    path.join(__dirname, 'cli', 'config.spec.js'),
+    path.join(__dirname, 'cli', 'run.spec.js')
+  ];
+  const resolvedTestFiles = testFiles.map(require.resolve);
 
-  describe('.unloadFile()', function() {
+  describe('#addFile', function() {
+    it('should add the given file to the files array', function() {
+      const mocha = new Mocha(opts);
+      mocha.addFile(__filename);
+      expect(mocha.files, 'to have length', 1).and('to contain', __filename);
+    });
+
+    it('should be chainable', function() {
+      const mocha = new Mocha(opts);
+      expect(mocha.addFile(__filename), 'to be', mocha);
+    });
+  });
+
+  describe('#loadFiles', function() {
+    it('should load all files from the files array', function() {
+      const mocha = new Mocha(opts);
+
+      testFiles.forEach(mocha.addFile, mocha);
+      mocha.loadFiles();
+      expect(require.cache, 'to have keys', resolvedTestFiles);
+    });
+
+    it('should execute the optional callback if given', function() {
+      const mocha = new Mocha(opts);
+      expect(cb => {
+        mocha.loadFiles(cb);
+      }, 'to call the callback');
+    });
+  });
+
+  describe('.unloadFile', function() {
     it('should unload a specific file from cache', function() {
       const resolvedFilePath = require.resolve(__filename);
-
       require(__filename);
       expect(require.cache, 'to have key', resolvedFilePath);
 
@@ -19,23 +54,19 @@ describe('Mocha', function() {
     });
   });
 
-  describe('.unloadFiles()', function() {
+  describe('#unloadFiles', function() {
     it('should unload all test files from cache', function() {
       const mocha = new Mocha(opts);
-      let resolvedTestFiles;
-      const testFiles = [
-        __filename,
-        path.join(__dirname, 'cli', 'config.spec.js'),
-        path.join(__dirname, 'cli', 'run.spec.js')
-      ];
 
       testFiles.forEach(mocha.addFile, mocha);
       mocha.loadFiles();
-      resolvedTestFiles = testFiles.map(require.resolve);
-      expect(require.cache, 'to have keys', resolvedTestFiles);
-
       mocha.unloadFiles();
       expect(require.cache, 'not to have keys', resolvedTestFiles);
     });
+
+    it('should be chainable', function() {
+      const mocha = new Mocha(opts);
+      expect(mocha.unloadFiles(), 'to be', mocha);
+    });
   });
 });