easy_events: make event data static

First: this function is only used in debug-builds and not in
release/real builds. It is used to drive tests using the event-based
API.

A pointer to the local struct is passed to CURLMOPT_TIMERDATA, but the
CURLMOPT_TIMERFUNCTION calback can in fact be called even after this
funtion returns, namely when curl_multi_remove_handle() is called.

Reported-by: Brian Carpenter