Now that test.pri adds source files and headers by default, the .pro files
of our tests need to append to SOURCES and HEADERS to ensure everything
test.pri adds to SOURCES and HEADERS are kept around.
The string test for PacketDataStream uses the string
Bærtur -- but encoded as Latin1.
Keep the string -- it's as good as any -- but use UTF-8 instead,
to make clang happy.
Fixesmumble-voip/mumble#2978