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/unionWith.test.js

@@ -0,0 +1,24 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import unionWith from '../unionWith.js';
+
+describe('unionWith', function() {
+  it('should work with a `comparator`', function() {
+    var objects = [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }],
+        others = [{ 'x': 1, 'y': 1 }, { 'x': 1, 'y': 2 }],
+        actual = unionWith(objects, others, lodashStable.isEqual);
+
+    assert.deepStrictEqual(actual, [objects[0], objects[1], others[0]]);
+  });
+
+  it('should output values from the first possible array', function() {
+    var objects = [{ 'x': 1, 'y': 1 }],
+        others = [{ 'x': 1, 'y': 2 }];
+
+    var actual = unionWith(objects, others, function(a, b) {
+      return a.x == b.x;
+    });
+
+    assert.deepStrictEqual(actual, [{ 'x': 1, 'y': 1 }]);
+  });
+});