preproc: enhance and extend tests * t/preproc-demo.sh: New test, a "demo" of how the new pre-processing feature could be used in a real-world package. * t/preproc-errmsg.sh: New test, check that error messages remain useful when the new pre-processing features are involved. * t/preproc-reldir.sh: Split up ... * t/preproc-basics.sh, t/preproc-c-compile.sh: ... into these two tests, with some refactorings, clean-up and enhancements. * t/list-of-tests.mk: Adjust. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>