testing: use shell scripts in testing framework

There is no assurance that tests will fail when something is wrong (e.g. when they test something and then print a message but not exit with non-zero return code). However, when some module fails, test will fail too (thanks to sh -e).

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@61658 15284696-431f-4ddb-bdfa-cd5b030d7da7