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/chroot.py

@@ -7,26 +7,24 @@ http = TestHTTP()
 
 
 def check_chroot():
-    available = option.available
-
-    resp = http.put(
-        url='/config',
-        sock_type='unix',
-        addr=f'{option.temp_dir}/control.unit.sock',
-        body=json.dumps(
-            {
-                "listeners": {"*:7080": {"pass": "routes"}},
-                "routes": [
-                    {
-                        "action": {
-                            "share": option.temp_dir,
-                            "chroot": option.temp_dir,
+    return (
+        'success'
+        in http.put(
+            url='/config',
+            sock_type='unix',
+            addr=f'{option.temp_dir}/control.unit.sock',
+            body=json.dumps(
+                {
+                    "listeners": {"*:7080": {"pass": "routes"}},
+                    "routes": [
+                        {
+                            "action": {
+                                "share": option.temp_dir,
+                                "chroot": option.temp_dir,
+                            }
                         }
-                    }
-                ],
-            }
-        ),
+                    ],
+                }
+            ),
+        )['body']
     )
-
-    if 'success' in resp['body']:
-        available['features']['chroot'] = True