Conformance tests: classes_override.py should allow highlighting the decorator

[alvov26]

While working on support for this test in PyCharm, I noticed that it requires highlighting the function signature instead of the decorator (which is how PyCharm currently handles it).
For example:

    @staticmethod
    @override
    def static_method1() -> int:  # E: no matching signature in ancestor
        return 1

    @classmethod
    @override
    def class_method1(cls) -> int:  # E: no matching signature in ancestor
        return 1

This would be easy to change on our end, but I believe allowing the option to highlight the decorator would be preferable for several reasons:

1. Highlighting the function signature may overlap with other checks, such as PyCharm inspections or other linters.
2. If the decorator is removed, the error no longer applies, so highlighting the decorator itself makes sense.
3. Other conformance tests already allow highlighting decorators. For example:

• qualifiers_final_decorator.py

@final  # E[func]: not allowed on non-method function.
def func1() -> int:  # E[func]
    return 0

• overloads_definitions.py

@overload  # E[func2]
def func2(x: int) -> int:  # E[func2]: no implementation
    ...

If this change makes sense, I’d be happy to submit a PR to update the test.

[JelleZijlstra]

Makes sense to me, feel free to send a PR.

The exact placement of errors isn't a strict requirement, and we've mostly done the placement on the basis of the behavior of type checkers we're already testing. If it makes sense to use a different line to show the error I'm happy to allow it.