test: replace concatenation with template literals

Replace string concatenation in test/async-hooks/test-signalwrap.js
with template literals.

PR-URL: #14295
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/async-hooks/test-signalwrap.js

@@ -22,7 +22,7 @@ assert.strictEqual(typeof signal1.triggerAsyncId, 'number');
 checkInvocations(signal1, { init: 1 }, 'when SIGUSR2 handler is set up');
 
 let count = 0;
-exec('kill -USR2 ' + process.pid);
+exec(`kill -USR2 ${process.pid}`);
 
 let signal2;
 
@@ -36,7 +36,7 @@ function onsigusr2() {
  ' signal1: when first SIGUSR2 handler is called for the first time');
 
  // trigger same signal handler again
- exec('kill -USR2 ' + process.pid);
+ exec(`kill -USR2 ${process.pid}`);
  } else {
  // second invocation
  checkInvocations(
@@ -61,7 +61,7 @@ function onsigusr2() {
  signal2, { init: 1 },
  'signal2: when second SIGUSR2 handler is setup');
 
- exec('kill -USR2 ' + process.pid);
+ exec(`kill -USR2 ${process.pid}`);
  }
 }