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/unix_abstract.py

@@ -7,19 +7,19 @@ http = TestHTTP()
 
 
 def check_unix_abstract():
-    available = option.available
-
-    resp = http.put(
-        url='/config',
-        sock_type='unix',
-        addr=f'{option.temp_dir}/control.unit.sock',
-        body=json.dumps(
-            {
-                "listeners": {"unix:@sock": {"pass": "routes"}},
-                "routes": [],
-            }
-        ),
+    return (
+        'success'
+        in http.put(
+            url='/config',
+            sock_type='unix',
+            addr=f'{option.temp_dir}/control.unit.sock',
+            body=json.dumps(
+                {
+                    "listeners": {
+                        f'unix:@{option.temp_dir}/sock': {"pass": "routes"}
+                    },
+                    "routes": [],
+                }
+            ),
+        )['body']
     )
-
-    if 'success' in resp['body']:
-        available['features']['unix_abstract'] = True