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/partialRight.js

@@ -0,0 +1,18 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import partialRight from '../partialRight.js';
+import mergeWith from '../mergeWith.js';
+
+describe('partialRight', function() {
+  it('should work as a deep `_.defaults`', function() {
+    var object = { 'a': { 'b': 2 } },
+        source = { 'a': { 'b': 3, 'c': 3 } },
+        expected = { 'a': { 'b': 2, 'c': 3 } };
+
+    var defaultsDeep = partialRight(mergeWith, function deep(value, other) {
+      return lodashStable.isObject(value) ? mergeWith(value, other, deep) : value;
+    });
+
+    assert.deepStrictEqual(defaultsDeep(object, source), expected);
+  });
+});