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_static_mount.py

@@ -5,15 +5,12 @@ from pathlib import Path
 import pytest
 from unit.applications.proto import TestApplicationProto
 
+prerequisites = {'features': {'chroot': True}, 'privileged_user': True}
+
 
 class TestStaticMount(TestApplicationProto):
-    prerequisites = {'features': ['chroot']}
-
     @pytest.fixture(autouse=True)
-    def setup_method_fixture(self, is_su, temp_dir):
-        if not is_su:
-            pytest.skip('requires root')
-
+    def setup_method_fixture(self, temp_dir):
         os.makedirs(f'{temp_dir}/assets/dir/mount')
         os.makedirs(f'{temp_dir}/assets/dir/dir')
         os.makedirs(f'{temp_dir}/assets/mount')