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/toPlainObject.js

@@ -0,0 +1,30 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { args } from './utils.js';
+import toPlainObject from '../toPlainObject.js';
+
+describe('toPlainObject', function() {
+  it('should flatten inherited string keyed properties', function() {
+    function Foo() {
+      this.b = 2;
+    }
+    Foo.prototype.c = 3;
+
+    var actual = lodashStable.assign({ 'a': 1 }, toPlainObject(new Foo));
+    assert.deepStrictEqual(actual, { 'a': 1, 'b': 2, 'c': 3 });
+  });
+
+  it('should convert `arguments` objects to plain objects', function() {
+    var actual = toPlainObject(args),
+        expected = { '0': 1, '1': 2, '2': 3 };
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should convert arrays to plain objects', function() {
+    var actual = toPlainObject(['a', 'b', 'c']),
+        expected = { '0': 'a', '1': 'b', '2': 'c' };
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});