run-file: failing test

The problem seems to be that I can't redefine an already-defined
procedure 'call-with-input-file' to mock its behaviour.
run-file: passing test for running a (dummy) file

Now using a new implementation of 'stub', which can now reach into any
module and change any variable value inside. Obviously, highly specific
to Guile.
main: exit code should be number of failed tests
Add user story
Merge branch 'devel'