Skip test if sanity_patterns is undefined

Author: Vasileios Karakasis

reframe/frontend/loader.py

@@ -81,7 +81,8 @@ def _validate_check(self, check):
 
         name = type(check).__name__
         checkfile = os.path.relpath(inspect.getfile(type(check)))
-        required_attrs = ['valid_systems', 'valid_prog_environs']
+        required_attrs = ['sanity_patterns',
+                          'valid_systems', 'valid_prog_environs']
         for attr in required_attrs:
             if getattr(check, attr) is None:
                 getlogger().warning(