In theory, the channel state could arrive to the state listener
before the arrays of channel states updated, causing a flakey test.

This fixes that.

Some test runners were picking it up as a test in its own right,
which was then causing JUnit ti complain as it contains no test
case methods.

By making it abstract, it allows us to extend it for protocol testing
but not make it runnable in its own right.

State transitions could happen very quickly, meaning that channel.state
may have proceeded at the point we assert the channel has attached etc.