Tests: get rid of classes in test files.

Class usage came from the unittest framework and it was always redundant
after migration to the pytest.  This commit removes classes from files
containing tests to make them more readable and understandable.

test/test_asgi_application_unix_abstract.py

@@ -1,22 +1,21 @@
 from packaging import version
-from unit.applications.lang.python import TestApplicationPython
+from unit.applications.lang.python import ApplicationPython
 
 prerequisites = {
     'modules': {'python': lambda v: version.parse(v) >= version.parse('3.5')},
     'features': {'unix_abstract': True},
 }
 
+client = ApplicationPython(load_module='asgi')
 
-class TestASGIApplicationUnixAbstract(TestApplicationPython):
-    load_module = 'asgi'
 
-    def test_asgi_application_unix_abstract(self):
-        self.load('empty')
+def test_asgi_application_unix_abstract():
+    client.load('empty')
 
-        addr = '\0sock'
-        assert 'success' in self.conf(
-            {f"unix:@{addr[1:]}": {"pass": "applications/empty"}},
-            'listeners',
-        )
+    addr = '\0sock'
+    assert 'success' in client.conf(
+        {f"unix:@{addr[1:]}": {"pass": "applications/empty"}},
+        'listeners',
+    )
 
-        assert self.get(sock_type='unix', addr=addr)['status'] == 200
+    assert client.get(sock_type='unix', addr=addr)['status'] == 200