test: replace string concat in test-fs-watchfile.js

PR-URL: #14287
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-fs-watchfile.js

@@ -67,7 +67,7 @@ fs.watchFile(enoentFile, {interval: 0}, common.mustCall(function(curr, prev) {
 // Watch events should callback with a filename on supported systems.
 // Omitting AIX. It works but not reliably.
 if (common.isLinux || common.isOSX || common.isWindows) {
-  const dir = common.tmpDir + '/watch';
+  const dir = path.join(common.tmpDir, 'watch');
 
   fs.mkdir(dir, common.mustCall(function(err) {
     if (err) assert.fail(err);
@@ -79,7 +79,7 @@ if (common.isLinux || common.isOSX || common.isWindows) {
     }));
 
     const interval = setInterval(() => {
-      fs.writeFile(`${dir}/foo.txt`, 'foo', common.mustCall(function(err) {
+      fs.writeFile(path.join(dir, 'foo.txt'), 'foo', common.mustCall((err) => {
         if (err) assert.fail(err);
       }));
     }, 1);