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/isObject.test.js

@@ -0,0 +1,53 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { args, slice, document, body, symbol, falsey, stubFalse, realm } from './utils.js';
+import isObject from '../isObject.js';
+
+describe('isObject', function() {
+  it('should return `true` for objects', function() {
+    assert.strictEqual(isObject(args), true);
+    assert.strictEqual(isObject([1, 2, 3]), true);
+    assert.strictEqual(isObject(Object(false)), true);
+    assert.strictEqual(isObject(new Date), true);
+    assert.strictEqual(isObject(new Error), true);
+    assert.strictEqual(isObject(_), true);
+    assert.strictEqual(isObject(slice), true);
+    assert.strictEqual(isObject({ 'a': 1 }), true);
+    assert.strictEqual(isObject(Object(0)), true);
+    assert.strictEqual(isObject(/x/), true);
+    assert.strictEqual(isObject(Object('a')), true);
+
+    if (document) {
+      assert.strictEqual(isObject(body), true);
+    }
+    if (Symbol) {
+      assert.strictEqual(isObject(Object(symbol)), true);
+    }
+  });
+
+  it('should return `false` for non-objects', function() {
+    var values = falsey.concat(true, 1, 'a', symbol),
+        expected = lodashStable.map(values, stubFalse);
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? isObject(value) : isObject();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with objects from another realm', function() {
+    if (realm.element) {
+      assert.strictEqual(isObject(realm.element), true);
+    }
+    if (realm.object) {
+      assert.strictEqual(isObject(realm.boolean), true);
+      assert.strictEqual(isObject(realm.date), true);
+      assert.strictEqual(isObject(realm.function), true);
+      assert.strictEqual(isObject(realm.number), true);
+      assert.strictEqual(isObject(realm.object), true);
+      assert.strictEqual(isObject(realm.regexp), true);
+      assert.strictEqual(isObject(realm.string), true);
+    }
+  });
+});