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')
 });
 
-letcatchedServername;
 constpair=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
 constsslHello=fixtures.readSync('google_ssl_hello.bin');
 
 pair.encrypted.write(sslHello);
-
-process.on('exit',function(){
-assert.strictEqual('www.google.com',catchedServername);
-});