test: fix running fastify.test.js with node v8 (#3317)

`http.request(url, options, cb)` didn't exist until node v10.

Author: trentm

test/instrumentation/modules/fastify/fastify.test.js

@@ -101,11 +101,11 @@ test('captureBody', function (t) {
     // an argument to the callback
     const port = fastify.server.address().port
     const cReq = http.request(
-      'http://localhost:' + port + '/postSomeData',
       {
         method: 'POST',
         hostname: 'localhost',
         port,
+        path: '/postSomeData',
         headers: {
           'Content-Type': 'application/json',
           'Content-Length': Buffer.byteLength(postData)