Don't recreate window very rapidly in test case

Something in the QPA or with hardware GL in macOS must have changed
that now makes this scenario crash at random. Let's bypass it by
just creating a single context and window for the whole set of
tests.

Task-number: QTBUG-58107
Change-Id: Ib9a9daf2fe8502ab99c280edf46c007d44a5a1b7
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>