The best way to test an IP core

BFMs abstract the simulation so that instead of worries about wiggling ports you can for example perform an Avalon read transaction and let the BFM do the wiggling for you. There are some examples with the BFM documentation for Quartus.

Does your counter have Avalon interfaces for you to be able to do things like write an initial value, read it back, start/stop, etc.... If it's just a plain counter then a testbench should be sufficient since there is no way to extract a generic interface like that (Qsys uses conduit BFMs for that but you are wiggling wires at that point).