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_static_fallback.py

@@ -6,8 +6,6 @@ from unit.applications.proto import TestApplicationProto
 
 
 class TestStaticFallback(TestApplicationProto):
-    prerequisites = {}
-
     @pytest.fixture(autouse=True)
     def setup_method_fixture(self, temp_dir):
         assets_dir = f'{temp_dir}/assets'