Add hint for qWait usage

usage of qWait in tests is prone to flakiness, especially
when used as a workaround for a race condition. Discourage
its use; detail in the Early Warning System wiki offers
suggestions for better alternatives.

Task-number: QTQAINFRA-7156
Change-Id: I836864d80f237218b36ce725285c0cb2f68eee5b
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Audun Sutterud <audun.sutterud@qt.io>