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/now.js

@@ -0,0 +1,26 @@
+import assert from 'assert';
+import { _, stubA } from './utils.js';
+
+describe('now', function() {
+  it('should return the number of milliseconds that have elapsed since the Unix epoch', function(done) {
+    var stamp = +new Date,
+        actual = _.now();
+
+    assert.ok(actual >= stamp);
+
+    setTimeout(function() {
+      assert.ok(_.now() > actual);
+      done();
+    }, 32);
+  });
+
+  it('should work with mocked `Date.now`', function() {
+    var now = Date.now;
+    Date.now = stubA;
+
+    var actual = _.now();
+    Date.now = now;
+
+    assert.strictEqual(actual, 'a');
+  });
+});