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Add test files for each Python module

For now they only test that the corresponding module can be imported.
Move fatal function to separate module
Add test for util.fatal
Make fatal_run_process public

Move it to util.py and add tests for it.
Add tests for prereq.check_prereq
Add simple test of clone operation
Make it convenient to comment out output supressing for executed processes
Add util.chdir_and_back context manager
Add a Config class with dump/load functionality