This is in preparation of adding -Wzero-as-null-pointer-constant (or similar)
to the headers check.

Since QtTestLib has a lot of templates and macros, not all uses of 0 as nullptr
might have been detected by the headersclean check.

Task-number: QTBUG-45291
Change-Id: I21e9d8822e3a708010938e8d5ef2fd42ae6c8c68
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>