ssh -t dev.toolforge.org -L 3306:tools.db.svc.wikimedia.cloud:3306"

Sometimes it  takes a few tries to find the right ssh incantation, but port tunneling is definitely something that works.  One thing you might try is running ssh in verbose mode (ssh -v, or -vv, or -vvv to get increasingly more detailed logging).  A lot of it is low-level details which may not mean anything, but it's worth a shot to see if anything jumps out at you.

Another good debugging tool is netcat (i.e. the "nc" command).  You can set to listen for connections on your VPS server (on some arbitrary port) and then try to connect to it.  That should at least tell you if you're getting as far as executing a TCP handshake through your tunnel.