Avoid calling curl from tests

Adding so I don't forget -- we currently invoke curl in two tests which implicitly pulls it as a dependency:

$ ag "curl " test
test/parallel/test-http-304.js
13:  childProcess.exec('curl -i http://127.0.0.1:' + common.PORT + '/',

test/parallel/test-http-curl-chunk-problem.js
19:  console.log('making curl request');
20:  var cmd = 'curl http://127.0.0.1:' + common.PORT + '/ | ' +

Suggesting we replace this with.. javascript?