add plugins option to browser tests

used to pass your own testsBundle.pluginIds to the test runner

Original commit: elastic/kibana-plugin-helpers@d370d74bd4

packages/kbn-plugin-helpers/bin/plugin-helpers.js

@@ -34,6 +34,7 @@ program
   .command('test:browser')
   .description('Run the browser tests in a real web browser')
   .option('--dev', 'Enable dev mode, keeps the test server running')
+  .option('-p, --plugins <plugin-ids>', 'Manually specify which plugins\' test bundles to run')
   .on('--help', docs('test/browser'))
   .action(run('test/browser'));