qtqa shared tests should be split into pre-build, post-build tests

The qtqa shared autotests under tests/shared currently consist of some tests which need Qt to be built before they are run (e.g. symbol checker), and some tests which don't (e.g. license header checker).

Because of this, we're running them all after Qt is built, but this is quite inefficient for those tests which don't need Qt built. In particular, it is quite easy to get license headers wrong, and also easy to check the license headers, but the CI system will take a long time to get around to running the license header test.

We should split the tests into two parts:

• pre-build: these tests do not need Qt to be compiled (though they may depend on Qt to be configured, so that qmake is usable). Note this implies the tests shouldn't be Qt C++ programs, e.g. they could be implemented in perl (as the current license header test is). Generally these are static checks, parsing the source code.
• post-build: these tests need Qt to be compiled before running. They can be Qt C++ programs. Generally these are checking the binaries somehow.

The pre-build tests can then be moved so that they are done in between the `configure' and `make' steps in qtmod_test.pl. (Or they could even be moved to a separate stage, like the "linux static check" stage we used to have)