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_status_tls.py

@@ -1,10 +1,10 @@
 from unit.applications.tls import TestApplicationTLS
 from unit.status import Status
 
+prerequisites = {'modules': {'openssl': 'any'}}
+
 
 class TestStatusTLS(TestApplicationTLS):
-    prerequisites = {'modules': {'openssl': 'any'}}
-
     def test_status_tls_requests(self):
         self.certificate()