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_ruby_isolation.py

@@ -1,30 +1,26 @@
-import pytest
 from unit.applications.lang.ruby import TestApplicationRuby
-from unit.option import option
+
+prerequisites = {'modules': {'ruby': 'any'}, 'features': {'isolation': True}}
 
 
 class TestRubyIsolation(TestApplicationRuby):
-    prerequisites = {'modules': {'ruby': 'any'}, 'features': ['isolation']}
-
-    def test_ruby_isolation_rootfs(self, is_su, temp_dir):
-        isolation_features = option.available['features']['isolation'].keys()
-
-        if not is_su:
-            if not 'unprivileged_userns_clone' in isolation_features:
-                pytest.skip('requires unprivileged userns or root')
-
-            if 'user' not in isolation_features:
-                pytest.skip('user namespace is not supported')
-
-            if 'mnt' not in isolation_features:
-                pytest.skip('mnt namespace is not supported')
-
-            if 'pid' not in isolation_features:
-                pytest.skip('pid namespace is not supported')
-
+    def test_ruby_isolation_rootfs(self, is_su, require, temp_dir):
         isolation = {'rootfs': temp_dir}
 
         if not is_su:
+            require(
+                {
+                    'features': {
+                        'isolation': [
+                            'unprivileged_userns_clone',
+                            'user',
+                            'mnt',
+                            'pid',
+                        ]
+                    }
+                }
+            )
+
             isolation['namespaces'] = {
                 'mount': True,
                 'credential': True,