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 <[email protected]>
Reviewed-By: David Cai <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Timothy Gu <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>

Author: jiangplus

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

@@ -32,7 +32,7 @@ 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' +