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/mean.test.js

@@ -0,0 +1,18 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { empties, stubNaN } from './utils.js';
+import mean from '../mean.js';
+
+describe('mean', function() {
+  it('should return the mean of an array of numbers', function() {
+    var array = [4, 2, 8, 6];
+    assert.strictEqual(mean(array), 5);
+  });
+
+  it('should return `NaN` when passing empty `array` values', function() {
+    var expected = lodashStable.map(empties, stubNaN),
+        actual = lodashStable.map(empties, mean);
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});