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/mapKeys-and-mapValues.js

@@ -0,0 +1,36 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, falsey, stubObject, noop } from './utils.js';
+
+describe('mapKeys and mapValues', function() {
+  lodashStable.each(['mapKeys', 'mapValues'], function(methodName) {
+    var func = _[methodName],
+        object = { 'a': 1, 'b': 2 };
+
+    it('`_.' + methodName + '` should iterate over own string keyed properties of objects', function() {
+      function Foo() {
+        this.a = 'a';
+      }
+      Foo.prototype.b = 'b';
+
+      var actual = func(new Foo, function(value, key) { return key; });
+      assert.deepStrictEqual(actual, { 'a': 'a' });
+    });
+
+    it('`_.' + methodName + '` should accept a falsey `object`', function() {
+      var expected = lodashStable.map(falsey, stubObject);
+
+      var actual = lodashStable.map(falsey, function(object, index) {
+        try {
+          return index ? func(object) : func();
+        } catch (e) {}
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('`_.' + methodName + '` should return a wrapped value when chaining', function() {
+      assert.ok(_(object)[methodName](noop) instanceof _);
+    });
+  });
+});