eventer: add test for concurrent Post/Register
eventer: add test for concurrent Post/Register

This test reports the race condition when run
using "go test -race".

maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum