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/strict-mode-checks.js

@@ -0,0 +1,22 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, isStrict, freeze } from './utils.js';
+
+describe('strict mode checks', function() {
+  lodashStable.each(['assign', 'assignIn', 'bindAll', 'defaults', 'defaultsDeep', 'merge'], function(methodName) {
+    var func = _[methodName],
+        isBindAll = methodName == 'bindAll';
+
+    it('`_.' + methodName + '` should ' + (isStrict ? '' : 'not ') + 'throw strict mode errors', function() {
+      var object = freeze({ 'a': undefined, 'b': function() {} }),
+          pass = !isStrict;
+
+      try {
+        func(object, isBindAll ? 'b' : { 'a': 1 });
+      } catch (e) {
+        pass = !pass;
+      }
+      assert.ok(pass);
+    });
+  });
+});