Test suite: Start testing our output for valid UTF-8.

Feed the output of `tshark -G <glossary>` to `iconv -f UTF-8`. Adjust a
couple of the Bluetooth dissectors and X11 keysyms accordingly.

Change-Id: I5b04dc3fa4734c8f0a795daf44bd398fe5ebc1bd
Reviewed-on: https://code.wireshark.org/review/6146
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>