The QMacStyle is not built as part of QtWidgets at the moment (see the
TODO in src/widgets/styles/styles.pri), so disable the test for now.
I've included a comment in styles.pri to re-enable the test once the
style is done.
Change-Id: Ia6ea130135586f107ba257bf2c6c2c7187a830b6
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>