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/reduceRight.js

@@ -0,0 +1,56 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice } from './utils.js';
+import reduceRight from '../reduceRight.js';
+
+describe('reduceRight', function() {
+  var array = [1, 2, 3];
+
+  it('should use the last element of a collection as the default `accumulator`', function() {
+    assert.strictEqual(reduceRight(array), 3);
+  });
+
+  it('should provide correct `iteratee` arguments when iterating an array', function() {
+    var args;
+
+    reduceRight(array, function() {
+      args || (args = slice.call(arguments));
+    }, 0);
+
+    assert.deepStrictEqual(args, [0, 3, 2, array]);
+
+    args = undefined;
+    reduceRight(array, function() {
+      args || (args = slice.call(arguments));
+    });
+
+    assert.deepStrictEqual(args, [3, 2, 1, array]);
+  });
+
+  it('should provide correct `iteratee` arguments when iterating an object', function() {
+    var args,
+        object = { 'a': 1, 'b': 2 },
+        isFIFO = lodashStable.keys(object)[0] == 'a';
+
+    var expected = isFIFO
+      ? [0, 2, 'b', object]
+      : [0, 1, 'a', object];
+
+    reduceRight(object, function() {
+      args || (args = slice.call(arguments));
+    }, 0);
+
+    assert.deepStrictEqual(args, expected);
+
+    args = undefined;
+    expected = isFIFO
+      ? [2, 1, 'a', object]
+      : [1, 2, 'b', object];
+
+    reduceRight(object, function() {
+      args || (args = slice.call(arguments));
+    });
+
+    assert.deepStrictEqual(args, expected);
+  });
+});