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_asgi_application_unix_abstract.py

@@ -1,14 +1,13 @@
 from packaging import version
 from unit.applications.lang.python import TestApplicationPython
 
+prerequisites = {
+    'modules': {'python': lambda v: version.parse(v) >= version.parse('3.5')},
+    'features': {'unix_abstract': True},
+}
+
 
 class TestASGIApplicationUnixAbstract(TestApplicationPython):
-    prerequisites = {
-        'modules': {
-            'python': lambda v: version.parse(v) >= version.parse('3.5')
-        },
-        'features': ['unix_abstract'],
-    }
     load_module = 'asgi'
 
     def test_asgi_application_unix_abstract(self):