Run positive/negative qtests with the same CLI drivers

Currently tests that should fail need to run with some variant of NegativeCliDriver which expects that the return code for a query is non-zero. Alternatively, we could include the error (if any) in the query output file itself and assert simply that the output didn't change. There are certain advantages in doing so:

• Simpler code and less classes to maintain;
• Possibility to mix positive/negative queries in the same .q file;
• Possibility to include multiple negative queries in the same .q file;
• No need for special directory structure for positive/negative tests;

The proposal above implies removing NegativeCliDriver and related code in favor of the regular CLI drivers.