Added saving of tracebacks to the 'messages.xml' files.

Fixed a typo ('use_partials') in 'ElectreDiscordance'.
Some fixes in output files in tests (ordering of elements + new format
of 'messages.xml' file).
Added simple script for launching tests for all the modules at once.

'comparables' taken from central profiles should be lists instead of
dicts ('ElectreConcordance', 'ElectreDiscordance',
'ElectreCredibility').

Minor changes in test files (input and output).