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_websockets.py

@@ -6,13 +6,12 @@ from packaging import version
 from unit.applications.lang.python import TestApplicationPython
 from unit.applications.websockets import TestApplicationWebsocket
 
+prerequisites = {
+    'modules': {'python': lambda v: version.parse(v) >= version.parse('3.5')}
+}
+
 
 class TestASGIWebsockets(TestApplicationPython):
-    prerequisites = {
-        'modules': {
-            'python': lambda v: version.parse(v) >= version.parse('3.5')
-        }
-    }
     load_module = 'asgi'
 
     ws = TestApplicationWebsocket()