[test] run the test in the same order when calling ./tests or ./tests test
Remove debug messages
Optimize the way we store None values.
Normalize the use of the Unknown exception

Lots of duplicate code, would be maybe better to expose error functions as well.
Debug statements
Fix dump for FS backend
Fix stupid (and time-consuming) typo.

This fix #3 and all the test now pass again.
Remove some dump statements in the tests
Improve the output graph of Irmin.dump