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.js

@@ -0,0 +1,75 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import sortBy from '../sortBy.js';
+
+describe('sortBy', function() {
+  var objects = [
+    { 'a': 'x', 'b': 3 },
+    { 'a': 'y', 'b': 4 },
+    { 'a': 'x', 'b': 1 },
+    { 'a': 'y', 'b': 2 }
+  ];
+
+  it('should sort in ascending order by `iteratee`', function() {
+    var actual = lodashStable.map(sortBy(objects, function(object) {
+      return object.b;
+    }), 'b');
+
+    assert.deepStrictEqual(actual, [1, 2, 3, 4]);
+  });
+
+  it('should use `_.identity` when `iteratee` is nullish', function() {
+    var array = [3, 2, 1],
+        values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant([1, 2, 3]));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? sortBy(array, value) : sortBy(array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    var actual = lodashStable.map(sortBy(objects.concat(undefined), 'b'), 'b');
+    assert.deepStrictEqual(actual, [1, 2, 3, 4, undefined]);
+  });
+
+  it('should work with an object for `collection`', function() {
+    var actual = sortBy({ 'a': 1, 'b': 2, 'c': 3 }, Math.sin);
+    assert.deepStrictEqual(actual, [3, 1, 2]);
+  });
+
+  it('should move `NaN`, nullish, and symbol values to the end', function() {
+    var symbol1 = Symbol ? Symbol('a') : null,
+        symbol2 = Symbol ? Symbol('b') : null,
+        array = [NaN, undefined, null, 4, symbol1, null, 1, symbol2, undefined, 3, NaN, 2],
+        expected = [1, 2, 3, 4, symbol1, symbol2, null, null, undefined, undefined, NaN, NaN];
+
+    assert.deepStrictEqual(sortBy(array), expected);
+
+    array = [NaN, undefined, symbol1, null, 'd', null, 'a', symbol2, undefined, 'c', NaN, 'b'];
+    expected = ['a', 'b', 'c', 'd', symbol1, symbol2, null, null, undefined, undefined, NaN, NaN];
+
+    assert.deepStrictEqual(sortBy(array), expected);
+  });
+
+  it('should treat number values for `collection` as empty', function() {
+    assert.deepStrictEqual(sortBy(1), []);
+  });
+
+  it('should coerce arrays returned from `iteratee`', function() {
+    var actual = sortBy(objects, function(object) {
+      var result = [object.a, object.b];
+      result.toString = function() { return String(this[0]); };
+      return result;
+    });
+
+    assert.deepStrictEqual(actual, [objects[0], objects[2], objects[1], objects[3]]);
+  });
+
+  it('should work as an iteratee for methods like `_.map`', function() {
+    var actual = lodashStable.map([[2, 1, 3], [3, 2, 1]], sortBy);
+    assert.deepStrictEqual(actual, [[1, 2, 3], [1, 2, 3]]);
+  });
+});