[SerialPort] bindable property tests fail

SerialPort code was ported to using bindable properties, but the actual unit-tests for the bindable properties fail.

That is not visible in the CI, because the tests do not run without setting special environment variables.

When running the tests locally (both Linux and Windows), I see:

FAIL!  : tst_QSerialPort::bindingsAndProperties() Compared values are not the same
   Actual   (dbProp.value())              : Data5
   Expected (QSerialPort::DataBits::Data8): Data8
   Loc: [qt5/qtserialport/tests/auto/qserialport/tst_qserialport.cpp(1285)]