Can we test that CasperJS is installed with something like this (or some other drush command I don't know about)?

```php
$casper = shell_exec('which casperjs');
if (!strpos($casper, 'casperjs')) {
  return drush_set_error('CASPERJS_TEST_FAILED', dt('CasperJS not installed.'));
}
$phantom = shell_exec('which phantomjs');
if (!strpos($output, 'phantomjs')) {
  $slimer = shell_exec('which slimerjs');
  if (!strpos($output, 'slimerjs')) {
    return drush_set_error('CASPERJS_TEST_FAILED', dt('Neither PhantomJS nor SlimerJS is installed.'));
  }
}
```