Add a new helper function to testlib

While fixing different auto-tests failing on macOS 15 (and probably above) due to our file-based keychain not working for us anymore, we introduced a fix which is in use by QtNetwork but also needed by several modules other than qtbase/QtNetwork. Rather than copying the code this way or another in other modules (in qtbase the function was inside network-helpers.h) we can introduce a new QTest::helperFunction and re-use it where needed.
After this function is in place, several tests previously 'patched' can start using it.