tests/seeyou: Use BytesIO instead of StringIO
seeyou: Added encoding property to Writer class
tests/xcsoar: Use BytesIO instead of StringIO
xcsoar: Added encoding property to Writer class
tests/flarmcfg: Use BytesIO instead of StringIO
flarmcfg: Added encoding property to Writer class
tests/igc: Use BytesIO instead of StringIO
igc: Added encoding property to Writer class