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_node_application.py

@@ -4,10 +4,10 @@ import pytest
 from unit.applications.lang.node import TestApplicationNode
 from unit.utils import waitforfiles
 
+prerequisites = {'modules': {'node': 'all'}}
+
 
 class TestNodeApplication(TestApplicationNode):
-    prerequisites = {'modules': {'node': 'all'}}
-
     def assert_basic_application(self):
         resp = self.get()
         assert resp['headers']['Content-Type'] == 'text/plain', 'basic header'