test: use template literals as appropriate

Replace string concatenation with template string literals in
test-graph.signal.js.

PR-URL: #14289
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.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>

test/async-hooks/test-graph.signal.js

@@ -16,20 +16,20 @@ hooks.enable();
 process.on('SIGUSR2', common.mustCall(onsigusr2, 2));
 
 let count = 0;
-exec('kill -USR2 ' + process.pid);
+exec(`kill -USR2 ${process.pid}`);
 
 function onsigusr2() {
   count++;
 
   if (count === 1) {
     // trigger same signal handler again
-    exec('kill -USR2 ' + process.pid);
+    exec(`kill -USR2 ${process.pid}`);
   } else {
     // install another signal handler
     process.removeAllListeners('SIGUSR2');
     process.on('SIGUSR2', common.mustCall(onsigusr2Again));
 
-    exec('kill -USR2 ' + process.pid);
+    exec(`kill -USR2 ${process.pid}`);
   }
 }