Make illegal to set a regular Python class attribute as `required`

Users should get an error when this is attempted. Otherwise the code below would be legal (and it probably shouldn't!)
```python
@rfm.simple_test
class MyTest(rfm.RunOnlyRegressionTest):
    ...
    my_attr = required

    @run_after('init')
    def my_hook(self):
        print(self.my_attr) # This would print the reframe.core.variables._UndefinedType object.
```