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/noConflict.js

@@ -0,0 +1,33 @@
+import assert from 'assert';
+import { oldDash, coverage, document, isModularize, realm, filePath } from './utils.js';
+import noConflict from '../noConflict.js';
+
+describe('noConflict', function() {
+  it('should return the `lodash` function', function() {
+    assert.strictEqual(noConflict(), oldDash);
+    assert.notStrictEqual(root._, oldDash);
+    root._ = oldDash;
+  });
+
+  it('should restore `_` only if `lodash` is the current `_` value', function() {
+    var object = root._ = {};
+    assert.strictEqual(noConflict(), oldDash);
+    assert.strictEqual(root._, object);
+    root._ = oldDash;
+  });
+
+  it('should work with a `root` of `this`', function() {
+    if (!coverage && !document && !isModularize && realm.object) {
+      var fs = require('fs'),
+          vm = require('vm'),
+          expected = {},
+          context = vm.createContext({ '_': expected, 'console': console }),
+          source = fs.readFileSync(filePath, 'utf8');
+
+      vm.runInContext(source + '\nthis.lodash = this._.noConflict()', context);
+
+      assert.strictEqual(context._, expected);
+      assert.ok(context.lodash);
+    }
+  });
+});