test: clean up string concat in dlopen-ping-pong

replace string concatenation with template literals.

PR-URL: #15820
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/addons/dlopen-ping-pong/test.js

@@ -11,7 +11,7 @@ const os = require('os');
 const bindingPath = require.resolve(`./build/${common.buildType}/binding`);
 process.dlopen(module, bindingPath,
                os.constants.dlopen.RTLD_NOW | os.constants.dlopen.RTLD_GLOBAL);
-module.exports.load(path.dirname(bindingPath) + '/ping.so');
+module.exports.load(`${path.dirname(bindingPath)}/ping.so`);
 assert.strictEqual(module.exports.ping(), 'pong');
 
 // Check that after the addon is loaded with