Is there an efficient easy way to test Drupal code while developing it?
As you're comfortable with Grunt, I've got a Gruntfile that I wrote that I use as a stub for most projects. You can find it here github.com/Chapabu/grunt-drupal-standards/. Just clone it to your project root and follow the installation instructions.

There are Drupal specific tasks there for doing to following:


PHP linting of any custom code
JS linting
CSS linting
Automatic clearing of theme registry when new templates are found
PHP_CodeSniffer tasks to ensure your code mets the Drupal Coding Standards


I need to make a few updates (i.e. remove Casper stuff, remove Compass tasks as I now use Libsass etc) but the tasks that are there should be enough to get you started.

There are also Grunt tasks for running PHPUnit tests or PHP Mess Detector (to name a couple of helpful suites) so it's pretty easy to add tasks to watches.