test: revise test-http-response-multi-content-length

Isolate the two test cases here a bit more and refactor code.

PR-URL: nodejs#32526
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-response-multi-content-length.js

@@ -3,45 +3,39 @@
 const common = require('../common');
 const http = require('http');
 const assert = require('assert');
-const Countdown = require('../common/countdown');
 
-const MAX_COUNT = 2;
+// TODO(@jasnell) At some point this should be refactored as the API should not
+// be allowing users to set multiple content-length values in the first place.
 
-const server = http.createServer((req, res) => {
-  const num = req.headers['x-num'];
-  // TODO(@jasnell) At some point this should be refactored as the API
-  // should not be allowing users to set multiple content-length values
-  // in the first place.
-  switch (num) {
-    case '1':
-      res.setHeader('content-length', [2, 1]);
-      break;
-    case '2':
-      res.writeHead(200, { 'content-length': [1, 2] });
-      break;
-    default:
-      assert.fail('should never get here');
-  }
-  res.end('ok');
-});
-
-const countdown = new Countdown(MAX_COUNT, () => server.close());
-
-server.listen(0, common.mustCall(() => {
-  for (let n = 1; n <= MAX_COUNT; n++) {
-    // This runs twice, the first time, the server will use
-    // setHeader, the second time it uses writeHead. In either
-    // case, the error handler must be called because the client
-    // is not allowed to accept multiple content-length headers.
+function test(server) {
+  server.listen(0, common.mustCall(() => {
     http.get(
-      { port: server.address().port, headers: { 'x-num': n } },
-      (res) => {
-        assert.fail('client allowed multiple content-length headers.');
-      }
+      { port: server.address().port },
+      () => { assert.fail('Client allowed multiple content-length headers.'); }
     ).on('error', common.mustCall((err) => {
-      assert(/^Parse Error/.test(err.message));
+      assert.ok(err.message.startsWith('Parse Error'), err.message);
       assert.strictEqual(err.code, 'HPE_UNEXPECTED_CONTENT_LENGTH');
-      countdown.dec();
+      server.close();
     }));
-  }
-}));
+  }));
+}
+
+// Test adding an extra content-length header using setHeader().
+{
+  const server = http.createServer((req, res) => {
+    res.setHeader('content-length', [2, 1]);
+    res.end('ok');
+  });
+
+  test(server);
+}
+
+// Test adding an extra content-length header using writeHead().
+{
+  const server = http.createServer((req, res) => {
+    res.writeHead(200, { 'content-length': [1, 2] });
+    res.end('ok');
+  });
+
+  test(server);
+}