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/isIndex.test.js

@@ -0,0 +1,34 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { MAX_SAFE_INTEGER, stubTrue, stubFalse } from './utils.js';
+import _isIndex from '../.internal/isIndex.js';
+
+describe('isIndex', function() {
+  var func = _isIndex;
+
+  it('should return `true` for indexes', function() {
+    if (func) {
+      var values = [[0], ['0'], ['1'], [3, 4], [MAX_SAFE_INTEGER - 1]],
+          expected = lodashStable.map(values, stubTrue);
+
+      var actual = lodashStable.map(values, function(args) {
+        return func.apply(undefined, args);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    }
+  });
+
+  it('should return `false` for non-indexes', function() {
+    if (func) {
+      var values = [['1abc'], ['07'], ['0001'], [-1], [3, 3], [1.1], [MAX_SAFE_INTEGER]],
+          expected = lodashStable.map(values, stubFalse);
+
+      var actual = lodashStable.map(values, function(args) {
+        return func.apply(undefined, args);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    }
+  });
+});