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_go_application.py

@@ -3,10 +3,10 @@ import re
 import pytest
 from unit.applications.lang.go import TestApplicationGo
 
+prerequisites = {'modules': {'go': 'all'}}
+
 
 class TestGoApplication(TestApplicationGo):
-    prerequisites = {'modules': {'go': 'all'}}
-
     @pytest.fixture(autouse=True)
     def setup_method_fixture(self, skip_alert):
         skip_alert(r'\[unit\] close\(\d+\) failed: Bad file descriptor')