test: improve coverage of lib/_http_outgoing.js

Refs: https://coverage.nodejs.org/coverage-36bb31be5f0b85a0/lib/_http_outgoing.js.html#L351
Refs: https://coverage.nodejs.org/coverage-36bb31be5f0b85a0/lib/_http_outgoing.js.html#L609

PR-URL: #38734
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-information-headers.js

@@ -1,5 +1,5 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const http = require('http');
 const Countdown = require('../common/countdown');
@@ -13,7 +13,7 @@ const server = http.createServer((req, res) => {
   // to call res.writeHead instead
   res._writeRaw('HTTP/1.1 102 Processing\r\n');
   res._writeRaw('Foo: Bar\r\n');
-  res._writeRaw('\r\n');
+  res._writeRaw('\r\n', common.mustCall());
   console.error('Server sending full response...');
   res.writeHead(200, {
     'Content-Type': 'text/plain',

test/parallel/test-http-mutable-headers.js

@@ -47,6 +47,7 @@ const s = http.createServer(common.mustCall((req, res) => {
       const exoticObj = Object.create(null);
       assert.deepStrictEqual(headers, exoticObj);
       assert.deepStrictEqual(res.getHeaderNames(), []);
+      assert.deepStrictEqual(res.getRawHeaderNames(), []);
       assert.deepStrictEqual(res.hasHeader('Connection'), false);
       assert.deepStrictEqual(res.getHeader('Connection'), undefined);