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/flip.test.js

@@ -0,0 +1,14 @@
+import assert from 'assert';
+import { slice } from './utils.js';
+import flip from '../flip.js';
+
+describe('flip', function() {
+  function fn() {
+    return slice.call(arguments);
+  }
+
+  it('should flip arguments provided to `func`', function() {
+    var flipped = flip(fn);
+    assert.deepStrictEqual(flipped('a', 'b', 'c', 'd'), ['d', 'c', 'b', 'a']);
+  });
+});