Add initial test files from lodash v4. (#4172)

* Install test dependencies.

* Add initial test files.

These files were created using a simplistic AST manipulator using `recast` to
preserve formatting. There's bound to be a huge chunk of errors, but this serves
as a good start. QUnit was replaced with Mocha, with ES2015 imports running via
`esm`.

As far as possible, QUnit-specific syntax has been replaced with Mocha's
`describe` and `it`, while the native Node.js `assert` module is used for
assertions. Files in the `test` directory ending in `.test.js` will be treated
as test files.

* Add initial passing files to test run.

test/functions.test.js

@@ -0,0 +1,22 @@
+import assert from 'assert';
+import { identity, noop } from './utils.js';
+import functions from '../functions.js';
+
+describe('functions', function() {
+  it('should return the function names of an object', function() {
+    var object = { 'a': 'a', 'b': identity, 'c': /x/, 'd': noop },
+        actual = functions(object).sort();
+
+    assert.deepStrictEqual(actual, ['b', 'd']);
+  });
+
+  it('should not include inherited functions', function() {
+    function Foo() {
+      this.a = identity;
+      this.b = 'b';
+    }
+    Foo.prototype.c = noop;
+
+    assert.deepStrictEqual(functions(new Foo), ['a']);
+  });
+});