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/setWith.js

@@ -0,0 +1,19 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { noop } from './utils.js';
+import setWith from '../setWith.js';
+
+describe('setWith', function() {
+  it('should work with a `customizer` callback', function() {
+    var actual = setWith({ '0': {} }, '[0][1][2]', 3, function(value) {
+      return lodashStable.isObject(value) ? undefined : {};
+    });
+
+    assert.deepStrictEqual(actual, { '0': { '1': { '2': 3 } } });
+  });
+
+  it('should work with a `customizer` that returns `undefined`', function() {
+    var actual = setWith({}, 'a[0].b.c', 4, noop);
+    assert.deepStrictEqual(actual, { 'a': [{ 'b': { 'c': 4 } }] });
+  });
+});