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.js

@@ -0,0 +1,35 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import mapKeys from '../mapKeys.js';
+
+describe('mapKeys', function() {
+  var array = [1, 2],
+      object = { 'a': 1, 'b': 2 };
+
+  it('should map keys in `object` to a new object', function() {
+    var actual = mapKeys(object, String);
+    assert.deepStrictEqual(actual, { '1': 1, '2': 2 });
+  });
+
+  it('should treat arrays like objects', function() {
+    var actual = mapKeys(array, String);
+    assert.deepStrictEqual(actual, { '1': 1, '2': 2 });
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    var actual = mapKeys({ 'a': { 'b': 'c' } }, 'b');
+    assert.deepStrictEqual(actual, { 'c': { 'b': 'c' } });
+  });
+
+  it('should use `_.identity` when `iteratee` is nullish', function() {
+    var object = { 'a': 1, 'b': 2 },
+        values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant({ '1': 1, '2': 2 }));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? mapKeys(object, value) : mapKeys(object);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});