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/unescape.js

@@ -0,0 +1,34 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import unescape from '../unescape.js';
+import escape from '../escape.js';
+
+describe('unescape', function() {
+  var escaped = '&<>"'/',
+      unescaped = '&<>"\'/';
+
+  escaped += escaped;
+  unescaped += unescaped;
+
+  it('should unescape entities in order', function() {
+    assert.strictEqual(unescape('&amp;lt;'), '&lt;');
+  });
+
+  it('should unescape the proper entities', function() {
+    assert.strictEqual(unescape(escaped), unescaped);
+  });
+
+  it('should handle strings with nothing to unescape', function() {
+    assert.strictEqual(unescape('abc'), 'abc');
+  });
+
+  it('should unescape the same characters escaped by `_.escape`', function() {
+    assert.strictEqual(unescape(escape(unescaped)), unescaped);
+  });
+
+  lodashStable.each(['&#96;', '&#x2F;'], function(entity) {
+    it('should not unescape the "' + entity + '" entity', function() {
+      assert.strictEqual(unescape(entity), entity);
+    });
+  });
+});