Tests: prerequisites checking reworked.

Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.

test/unit/applications/proto.py

@@ -1,9 +1,6 @@
 import os
-import re
-import time
 
 from unit.control import TestControl
-from unit.log import Log
 from unit.option import option