Disable TextCtrlTestCase::LongText() for non-MSW platforms.

At least under GTK SetMaxLength(), tested by this test, can't be used with
multiline text controls at all, so running it fails. Keep the test for MSW
only where SetMaxLength() works with controls of all kinds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76326 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775