Use casts to improve type checking for MockConsole

[trickeydan]

I'm guessing type: ignore is needed here because the typechecker doesn't know that backend._console is a MockConsole rather than the more general Console. Could we instead pull out the console into a local variable using a cast and refer to it directly?

console = cast(MockConsole, backend._console)
console.expects = ...

Even better, could we refactor so that the console object is created in this function and passed into the ArduinoUnoConsoleBackend constructor, rather than being created inside the constructor and having to be pulled out of a private attribute?

Originally posted by @kierdavis in https://github.com/j5api/j5/pull/306/files

[trickeydan]

Relevant: #404