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_chroot.py

@@ -5,10 +5,10 @@ import pytest
 from unit.applications.proto import TestApplicationProto
 from unit.option import option
 
+prerequisites = {'features': {'chroot': True}}
+
 
 class TestStaticChroot(TestApplicationProto):
-    prerequisites = {'features': ['chroot']}
-
     @pytest.fixture(autouse=True)
     def setup_method_fixture(self, temp_dir):
         os.makedirs(f'{temp_dir}/assets/dir')
@@ -62,9 +62,8 @@ class TestStaticChroot(TestApplicationProto):
         )
         assert self.get()['status'] != 200, 'share array bad'
 
-    def test_static_chroot_permission(self, is_su, temp_dir):
-        if is_su:
-            pytest.skip("does't work under root")
+    def test_static_chroot_permission(self, require, temp_dir):
+        require({'privileged_user': False})
 
         os.chmod(f'{temp_dir}/assets/dir', 0o100)
 
@@ -81,9 +80,8 @@ class TestStaticChroot(TestApplicationProto):
         assert 'success' in self.update_action("", ".$uri")
         assert self.get(url=self.test_path)['status'] == 200, 'empty relative'
 
-    def test_static_chroot_relative(self, is_su):
-        if is_su:
-            pytest.skip("Does't work under root.")
+    def test_static_chroot_relative(self, require):
+        require({'privileged_user': False})
 
         assert 'success' in self.update_action('.')
         assert self.get(url='/dir/file')['status'] == 403, 'relative chroot'