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/test_settings.py

@@ -6,10 +6,10 @@ import time
 import pytest
 from unit.applications.lang.python import TestApplicationPython
 
+prerequisites = {'modules': {'python': 'any'}}
+
 
 class TestSettings(TestApplicationPython):
-    prerequisites = {'modules': {'python': 'any'}}
-
     def sysctl(self):
         try:
             out = subprocess.check_output(