Partially re-enable Wireshark tests.

Qt requires Xrender and XKEYBOARD. Unfortunately the Ubuntu and Solaris
buildbots run the test script under Xtightvnc, which provides neither of
those. Only enable the tests on Windows and OS X for now.

Change-Id: I84bc28f810782b862b4dca8fc8df088a4919066d
Reviewed-on: https://code.wireshark.org/review/4430
Reviewed-by: Gerald Combs <gerald@wireshark.org>