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/unzipWith.js

@@ -0,0 +1,39 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice } from './utils.js';
+import unzipWith from '../unzipWith.js';
+import unzip from '../unzip.js';
+
+describe('unzipWith', function() {
+  it('should unzip arrays combining regrouped elements with `iteratee`', function() {
+    var array = [[1, 4], [2, 5], [3, 6]];
+
+    var actual = unzipWith(array, function(a, b, c) {
+      return a + b + c;
+    });
+
+    assert.deepStrictEqual(actual, [6, 15]);
+  });
+
+  it('should provide correct `iteratee` arguments', function() {
+    var args;
+
+    unzipWith([[1, 3, 5], [2, 4, 6]], function() {
+      args || (args = slice.call(arguments));
+    });
+
+    assert.deepStrictEqual(args, [1, 2]);
+  });
+
+  it('should perform a basic unzip when `iteratee` is nullish', function() {
+    var array = [[1, 3], [2, 4]],
+        values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant(unzip(array)));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? unzipWith(array, value) : unzipWith(array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});