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_conf_command.py

@@ -3,10 +3,10 @@ import ssl
 import pytest
 from unit.applications.tls import TestApplicationTLS
 
+prerequisites = {'modules': {'openssl': 'any'}}
+
 
 class TestTLSConfCommand(TestApplicationTLS):
-    prerequisites = {'modules': {'openssl': 'any'}}
-
     @pytest.fixture(autouse=True)
     def setup_method_fixture(self):
         self.certificate()