Motivation
NioEventLoopTest#testChannelsRegistered() fails intermittently due to
use of SingleThreadEventLoop#channelsRegistered() which is not
threadsafe and unreliable when called from outside the event loop.
Modifications
Add static registeredChannels method to NioEventLoopTest and
AbstractSingleThreadEventLoopTest to call from the tests via event loop
instead of directly.
Result
Hopefully fewer test failures
99a4cecec6