tests: Check the behaviour detailed in issue #22

https://github.com/bochecha/ibus-cangjie/issues/22
cangjie: clear input box on new keys after an error

fixes #22
Protect against failures to play the error bell

Playing an error sound is a great way to provide feedback to the user
when they make a mistake.

However, it might fail (e.g bug in the sound stack, or due to the
fragility of pycanberra), and it must not crash the engine.