test: replace concatenation with template literals

* test/parallel/test-stdout-close-catch.js

PR-URL: #14298
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-stdout-close-catch.js

@@ -6,9 +6,9 @@ const child_process = require('child_process');
 
 const testScript = path.join(common.fixturesDir, 'catch-stdout-error.js');
 
-const cmd = JSON.stringify(process.execPath) + ' ' +
-            JSON.stringify(testScript) + ' | ' +
-            JSON.stringify(process.execPath) + ' ' +
+const cmd = `${JSON.stringify(process.execPath)} ` +
+            `${JSON.stringify(testScript)} | ` +
+            `${JSON.stringify(process.execPath)} ` +
             '-pe "process.stdin.on(\'data\' , () => process.exit(1))"';
 
 const child = child_process.exec(cmd);