test: refactor test-tls-securepair-fiftharg

Assert the server name directly in the `SNICallback`,
since `common.mustCall()` already guarantees that the callback
is called exactly once, making `process.on('exit')` unnecessary.

PR-URL: #17836
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/parallel/test-tls-securepair-fiftharg.js

@@ -10,18 +10,13 @@ const sslcontext = tls.createSecureContext({
   key: fixtures.readSync('test_key.pem')
 });
 
-let catchedServername;
 const pair = tls.createSecurePair(sslcontext, true, false, false, {
-  SNICallback: common.mustCall(function(servername, cb) {
-    catchedServername = servername;
+  SNICallback: common.mustCall((servername, cb) => {
+    assert.strictEqual(servername, 'www.google.com');
   })
 });
 
 // captured traffic from browser's request to https://www.google.com
 const sslHello = fixtures.readSync('google_ssl_hello.bin');
 
 pair.encrypted.write(sslHello);
-
-process.on('exit', function() {
-  assert.strictEqual('www.google.com', catchedServername);
-});