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/groupBy.js

@@ -0,0 +1,68 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { LARGE_ARRAY_SIZE } from './utils.js';
+import groupBy from '../groupBy.js';
+
+describe('groupBy', function() {
+  var array = [6.1, 4.2, 6.3];
+
+  it('should transform keys by `iteratee`', function() {
+    var actual = groupBy(array, Math.floor);
+    assert.deepStrictEqual(actual, { '4': [4.2], '6': [6.1, 6.3] });
+  });
+
+  it('should use `_.identity` when `iteratee` is nullish', function() {
+    var array = [6, 4, 6],
+        values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant({ '4': [4], '6':  [6, 6] }));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? groupBy(array, value) : groupBy(array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    var actual = groupBy(['one', 'two', 'three'], 'length');
+    assert.deepStrictEqual(actual, { '3': ['one', 'two'], '5': ['three'] });
+  });
+
+  it('should only add values to own, not inherited, properties', function() {
+    var actual = groupBy(array, function(n) {
+      return Math.floor(n) > 4 ? 'hasOwnProperty' : 'constructor';
+    });
+
+    assert.deepStrictEqual(actual.constructor, [4.2]);
+    assert.deepStrictEqual(actual.hasOwnProperty, [6.1, 6.3]);
+  });
+
+  it('should work with a number for `iteratee`', function() {
+    var array = [
+      [1, 'a'],
+      [2, 'a'],
+      [2, 'b']
+    ];
+
+    assert.deepStrictEqual(groupBy(array, 0), { '1': [[1, 'a']], '2': [[2, 'a'], [2, 'b']] });
+    assert.deepStrictEqual(groupBy(array, 1), { 'a': [[1, 'a'], [2, 'a']], 'b': [[2, 'b']] });
+  });
+
+  it('should work with an object for `collection`', function() {
+    var actual = groupBy({ 'a': 6.1, 'b': 4.2, 'c': 6.3 }, Math.floor);
+    assert.deepStrictEqual(actual, { '4': [4.2], '6': [6.1, 6.3] });
+  });
+
+  it('should work in a lazy sequence', function() {
+    var array = lodashStable.range(LARGE_ARRAY_SIZE).concat(
+      lodashStable.range(Math.floor(LARGE_ARRAY_SIZE / 2), LARGE_ARRAY_SIZE),
+      lodashStable.range(Math.floor(LARGE_ARRAY_SIZE / 1.5), LARGE_ARRAY_SIZE)
+    );
+
+    var iteratee = function(value) { value.push(value[0]); return value; },
+        predicate = function(value) { return isEven(value[0]); },
+        actual = _(array).groupBy().map(iteratee).filter(predicate).take().value();
+
+    assert.deepEqual(actual, _.take(_.filter(lodashStable.map(groupBy(array), iteratee), predicate)));
+  });
+});