Can ComboBox get better tests?

[zepumph]

There were multiple bugs that came in related to PhET-iO + other features of Combo box (alt input and sound).

#859
#861
#862
#864

I would like to spend a few minutes trying to brainstorm how we may be able to write tests to help catch this kind of thing for our common code moving forward. @samreid, I'm implicating you for a 15 minute brainstorm (sorry).

This is also coming from https://github.com/phetsims/phet-io-wrappers/issues/632

[zepumph]

@samreid and I discussed this, and determined that we should focus on https://github.com/phetsims/phet-io/issues/1889 and ensuring that QA and devs are well set up to know important things to test, especially as new things come up like this. Closing