add test for shaker sort

Author: Pixelrick420

test/sorting/shakersort.spec.js

@@ -0,0 +1,5 @@
+var sortTestCase = require('./sort.testcase.js');
+var shakerSort =
+      require('../../src/sorting/shakersort.js').shakerSort;
+
+sortTestCase(shakerSort, 'Shaker sort');