tst_QSharedPointer can't create a pipe as the OS has too many files
open. Systems like macOS have a lower limit to these simultaneous files
open.
Task-number: QTBUG-60410
Change-Id: I21e89f992ada2a7d09b706522a05b5952f00ec33
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>