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/forIn-methods.js

@@ -0,0 +1,20 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _ } from './utils.js';
+
+describe('forIn methods', function() {
+  lodashStable.each(['forIn', 'forInRight'], function(methodName) {
+    var func = _[methodName];
+
+    it('`_.' + methodName + '` iterates over inherited string keyed properties', function() {
+      function Foo() {
+        this.a = 1;
+      }
+      Foo.prototype.b = 2;
+
+      var keys = [];
+      func(new Foo, function(value, key) { keys.push(key); });
+      assert.deepStrictEqual(keys.sort(), ['a', 'b']);
+    });
+  });
+});