How can I test my 500 error page works
Any chance you're out of disk space? That error shows that it can't create a file in /tmp which means either the permissions on /tmp are wrong, or potentially you're out of disk space.

c.f.: https://stackoverflow.com/questions/11997012/mysql-cant-create-write-to-file-tmp-sql-3c6-0-myi-errcode-2-what-does

As for the error page, if you just want to view the page, you can navigate to it directly... if you want to force a 500 error to happen, just change the db password in your .env to something that doesn't work.