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_upstreams_rr.py

@@ -5,10 +5,10 @@ import pytest
 from unit.applications.lang.python import TestApplicationPython
 from unit.option import option
 
+prerequisites = {'modules': {'python': 'any'}}
+
 
 class TestUpstreamsRR(TestApplicationPython):
-    prerequisites = {'modules': {'python': 'any'}}
-
     @pytest.fixture(autouse=True)
     def setup_method_fixture(self):
         assert 'success' in self.conf(