The TESTARGS environment variable, which both our CMake and QMake configurations support as a mechanism for passing arguments to runs of test programs built with QtTest, is only actually documented in the QMake docs., aside from an unexplained passing mention of it in the Best Practices doc.

We should document our CMake support for it, too. The wording in the QMake manual is a reasonable basis to work from, although it would need adapted to the possibility of running via ninja rather than make. There would be some sense in at least mentioning, in the QtTest docs (at least by reference), that Qt's own CMake and QMake build infrastructure support it.