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/unit/check/go.py

@@ -2,5 +2,4 @@ from unit.applications.lang.go import TestApplicationGo
 
 
 def check_go():
-    if TestApplicationGo.prepare_env('empty') is not None:
-        return True
+    return TestApplicationGo.prepare_env('empty') is not None