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/invertBy.js

@@ -0,0 +1,42 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import invertBy from '../invertBy.js';
+
+describe('invertBy', function() {
+  var object = { 'a': 1, 'b': 2, 'c': 1 };
+
+  it('should transform keys by `iteratee`', function() {
+    var expected = { 'group1': ['a', 'c'], 'group2': ['b'] };
+
+    var actual = invertBy(object, function(value) {
+      return 'group' + value;
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should use `_.identity` when `iteratee` is nullish', function() {
+    var values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant({ '1': ['a', 'c'], '2': ['b'] }));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? invertBy(object, value) : invertBy(object);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should only add multiple values to own, not inherited, properties', function() {
+    var object = { 'a': 'hasOwnProperty', 'b': 'constructor' },
+        expected = { 'hasOwnProperty': ['a'], 'constructor': ['b'] };
+
+    assert.ok(lodashStable.isEqual(invertBy(object), expected));
+  });
+
+  it('should return a wrapped value when chaining', function() {
+    var wrapped = _(object).invertBy();
+
+    assert.ok(wrapped instanceof _);
+    assert.deepEqual(wrapped.value(), { '1': ['a', 'c'], '2': ['b'] });
+  });
+});