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/rest.js

@@ -0,0 +1,49 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice, _ } from './utils.js';
+
+describe('rest', function() {
+  function fn(a, b, c) {
+    return slice.call(arguments);
+  }
+
+  it('should apply a rest parameter to `func`', function() {
+    var rest = _.rest(fn);
+    assert.deepStrictEqual(rest(1, 2, 3, 4), [1, 2, [3, 4]]);
+  });
+
+  it('should work with `start`', function() {
+    var rest = _.rest(fn, 1);
+    assert.deepStrictEqual(rest(1, 2, 3, 4), [1, [2, 3, 4]]);
+  });
+
+  it('should treat `start` as `0` for `NaN` or negative values', function() {
+    var values = [-1, NaN, 'a'],
+        expected = lodashStable.map(values, lodashStable.constant([[1, 2, 3, 4]]));
+
+    var actual = lodashStable.map(values, function(value) {
+      var rest = _.rest(fn, value);
+      return rest(1, 2, 3, 4);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should coerce `start` to an integer', function() {
+    var rest = _.rest(fn, 1.6);
+    assert.deepStrictEqual(rest(1, 2, 3), [1, [2, 3]]);
+  });
+
+  it('should use an empty array when `start` is not reached', function() {
+    var rest = _.rest(fn);
+    assert.deepStrictEqual(rest(1), [1, undefined, []]);
+  });
+
+  it('should work on functions with more than three parameters', function() {
+    var rest = _.rest(function(a, b, c, d) {
+      return slice.call(arguments);
+    });
+
+    assert.deepStrictEqual(rest(1, 2, 3, 4, 5), [1, 2, 3, [4, 5]]);
+  });
+});