Qt 6: Do not build tests and examples by default

In Qt 5 and current CMake based Qt 6, examples are built by default on most platforms (except ios and android). tests are built by default for a developer-build.

I suggest to change the default to never build examples and tests by default:

• Both take significant time to build
• Building all examples is mostly useful to see whether they actually compile. This can be nowadays catched by the CI
• Building autotests is useful for developers, but they typically want to build only some of them. Building and running all of them is again a job for the CI