fix(developer): raise error if virtual key in context string

Fixes #7880.

Virtual keys have never been allowed in context. This should be an error
in a keyboard. Note: virtual keys in output are officially unsupported
but still kinda sorta a little bit work in Windows -- but I doubt they
will ever be officially supported.

Also includes small side journey to tidy up names and constants for two
other tests (error_duplicate_group and error_duplicate_store).