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/sortBy-methods.js

@@ -0,0 +1,87 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _ } from './utils.js';
+
+describe('sortBy methods', function() {
+  lodashStable.each(['orderBy', 'sortBy'], function(methodName) {
+    var func = _[methodName];
+
+    function Pair(a, b, c) {
+      this.a = a;
+      this.b = b;
+      this.c = c;
+    }
+
+    var objects = [
+      { 'a': 'x', 'b': 3 },
+      { 'a': 'y', 'b': 4 },
+      { 'a': 'x', 'b': 1 },
+      { 'a': 'y', 'b': 2 }
+    ];
+
+    var stableArray = [
+      new Pair(1, 1, 1), new Pair(1, 2, 1),
+      new Pair(1, 1, 1), new Pair(1, 2, 1),
+      new Pair(1, 3, 1), new Pair(1, 4, 1),
+      new Pair(1, 5, 1), new Pair(1, 6, 1),
+      new Pair(2, 1, 2), new Pair(2, 2, 2),
+      new Pair(2, 3, 2), new Pair(2, 4, 2),
+      new Pair(2, 5, 2), new Pair(2, 6, 2),
+      new Pair(undefined, 1, 1), new Pair(undefined, 2, 1),
+      new Pair(undefined, 3, 1), new Pair(undefined, 4, 1),
+      new Pair(undefined, 5, 1), new Pair(undefined, 6, 1)
+    ];
+
+    var stableObject = lodashStable.zipObject('abcdefghijklmnopqrst'.split(''), stableArray);
+
+    it('`_.' + methodName + '` should sort multiple properties in ascending order', function() {
+      var actual = func(objects, ['a', 'b']);
+      assert.deepStrictEqual(actual, [objects[2], objects[0], objects[3], objects[1]]);
+    });
+
+    it('`_.' + methodName + '` should support iteratees', function() {
+      var actual = func(objects, ['a', function(object) { return object.b; }]);
+      assert.deepStrictEqual(actual, [objects[2], objects[0], objects[3], objects[1]]);
+    });
+
+    it('`_.' + methodName + '` should perform a stable sort (test in IE > 8 and V8)', function() {
+      lodashStable.each([stableArray, stableObject], function(value, index) {
+        var actual = func(value, ['a', 'c']);
+        assert.deepStrictEqual(actual, stableArray, index ? 'object' : 'array');
+      });
+    });
+
+    it('`_.' + methodName + '` should not error on nullish elements', function() {
+      try {
+        var actual = func(objects.concat(null, undefined), ['a', 'b']);
+      } catch (e) {}
+
+      assert.deepStrictEqual(actual, [objects[2], objects[0], objects[3], objects[1], null, undefined]);
+    });
+
+    it('`_.' + methodName + '` should work as an iteratee for methods like `_.reduce`', function() {
+      var objects = [
+        { 'a': 'x', '0': 3 },
+        { 'a': 'y', '0': 4 },
+        { 'a': 'x', '0': 1 },
+        { 'a': 'y', '0': 2 }
+      ];
+
+      var funcs = [func, lodashStable.partialRight(func, 'bogus')];
+
+      lodashStable.each(['a', 0, [0]], function(props, index) {
+        var expected = lodashStable.map(funcs, lodashStable.constant(
+          index
+            ? [objects[2], objects[3], objects[0], objects[1]]
+            : [objects[0], objects[2], objects[1], objects[3]]
+        ));
+
+        var actual = lodashStable.map(funcs, function(func) {
+          return lodashStable.reduce([props], func, objects);
+        });
+
+        assert.deepStrictEqual(actual, expected);
+      });
+    });
+  });
+});