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_tls.py

@@ -7,10 +7,10 @@ import pytest
 from unit.applications.tls import TestApplicationTLS
 from unit.option import option
 
+prerequisites = {'modules': {'python': 'any', 'openssl': 'any'}}
+
 
 class TestTLS(TestApplicationTLS):
-    prerequisites = {'modules': {'python': 'any', 'openssl': 'any'}}
-
     def add_tls(self, application='empty', cert='default', port=7080):
         assert 'success' in self.conf(
             {