Test the stats subsystem

[alessandropellegrini]

Introduce tests for the stats subsystem.

[Piccions]

Update: there are now tests for the statistics subsystem, but they are not complete.

• dumping statistics and parsing them via the python script is tested
• the format and most of the generated values in the textual report are tested
• per-GVT and per-thread values are only partially tested
• distributed consolidation of statistics is not tested (this depends also on Test suite enhancement #14).

Empirically, this all works, but at the moment not everything is covered by tests.