commands, retval, stdout & stderr are lists
if they don't have the same length, they are truncated to the smallest list
thereby silently dropping test cases
=> now a ValueError is raised preventing that
The testsuite now uses python's template module for string substitutions which
allows for a more natural substitution syntax known from the shell. Also, it
allows to run the substitutions multiple times, which is not possible with
string.format().
The heavy-lifting is now performed via a metaclass, which expands all variables
on the class creation.
