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/castArray.test.js

@@ -0,0 +1,23 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey } from './utils.js';
+import castArray from '../castArray.js';
+
+describe('castArray', function() {
+  it('should wrap non-array items in an array', function() {
+    var values = falsey.concat(true, 1, 'a', { 'a': 1 }),
+        expected = lodashStable.map(values, function(value) { return [value]; }),
+        actual = lodashStable.map(values, castArray);
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return array values by reference', function() {
+    var array = [1];
+    assert.strictEqual(castArray(array), array);
+  });
+
+  it('should return an empty array when no arguments are given', function() {
+    assert.deepStrictEqual(castArray(), []);
+  });
+});