testlib: added env QTEST_FUNCTION_TIMEOUT for single test function timeout

This is a regression with 5.5, it causes some customer test functions
crash. d3d10cf2

 introduced the 5 minutes timeout. The timeout could be
changed after this.

Task-number: QTBUG-52402
Change-Id: I8940bbde5004b4d888732b551274f079ddf20563
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>