Implement wait event processed logic

[matthiasgeihs]

Sometimes, especially in testing, we want to wait until a specific event has been processed by a watcher routine, as for example in https://github.com/hyperledger-labs/go-perun/pull/83#discussion_r665595730_ .

It would be useful to have some kind of waitProcessed logic on the watcher functionality.

[matthiasgeihs]

May be addressed by #172