preproc: enhance and extend tests

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>