test: simplify string concatenation

Replace string concatenation with template literals.
Updated `test/parallel/test-http-multi-line-headers.js`

PR-URL: #14278
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-http-multi-line-headers.js

@@ -11,7 +11,7 @@ const server = net.createServer(function(conn) {
   const response =
       'HTTP/1.1 200 OK\r\n' +
       'Connection: close\r\n' +
-      'Content-Length: ' + body.length + '\r\n' +
+      `Content-Length: ${body.length}\r\n` +
       'Content-Type: text/plain;\r\n' +
       ' x-unix-mode=0600;\r\n' +
       ' name="hello.txt"\r\n' +