Enable running auto-tests in qtdoc

After moving examples and tests from qtdeclarative to qtdoc (https://codereview.qt-project.org/#/c/232533/), the only "test" that runs is the license header test. No other test is run, neither on Windows, MacOS or Linux. This causes lost test coverage. See https://testresults.qt.io/coin/integration/qt/qtdoc/tasks/1531747172.

Coin has qtdoc hardcoded as a module that "has" no tests. Coin needs to be changed to attempt running tests in qtdoc. until that's fixed, we're running totally blind. This may potentially require changes in qtdoc across all branches, because coin's setting is global.

This must be fixed before the release of 5.12.