tests: add meta-tests related to the execution control

* zzz_tests/zzz_test_args.nit
* zzz_tests/zzz_test_envvar.nit
* zzz_tests/zzz_test_in.nit
* zzz_tests/zzz_test_post_proc.nit
* zzz_tests/zzz_test_write.nit

Signed-off-by: Jean Privat <jean@pryen.org>
tests: improve documentation

Signed-off-by: Jean Privat <jean@pryen.org>