Fix event_callback unittest
Fix event_callback unittest

Even after #67 was merged the event_callback unittest
fails occasionally.

Testing shows the only invocation of the user callback
function which is guaranteed happens after calling
drizzle_connect.

Thus the unittest has been loosened further to only check
that the call function has been invoked at least once.

sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux