statistics: fix return value of with_stats wrapper

The return value of functions, that are decorated with "@with_stats"
should not change the return value of the function.
tests/test_statistics: compare defaultdict with dict

You have to convert the defaultdict to a dict, before checking its
equality with a plain dict.
statistics: fix reinitialization
tests/test_statistics: add missing values to test_reinit
tests/test_statistics: fix typo
validator: collect some stats during validation