Disable comparison overlap checks in assert statements

Feature

In assert statements, allow comparisons that don't appear to overlap. These are common in test cases, and tend to generate false positives.

One way to implement this would be to filter out errors with the comparison-overlap error code in assert statements.

A potentially better way would be to not narrow down types in comparisons in assert statements, but this could be too complicated and ad hoc.

Example where we have a false positive:

# mypy: strict-equality
from enum import Enum

class MyEnum(Enum):
    X = 1
    Y = 2

class MyClass:
    attr: MyEnum = MyEnum.X

    def mutate(self) -> None:
        self.attr = MyEnum.Y

def test_foo() -> None:
    a = MyClass()
    assert a.attr == MyEnum.X
    a.mutate()
    assert a.attr == MyEnum.Y  # Error: Non-overlapping equality check

Pitch

These errors are often false positives, and they are somewhat frequent in test cases. The fix seems simple.

Hints

Here is an example of filtering errors adapted from mypy/plugins/default.py:

            with self.msg.filter_errors(
                filter_errors=lambda name, info: info.code != codes.TYPEDDICT_READONLY_MUTATED,
                save_filtered_errors=True,
            ):

Start by tracing how assert statements are checked and how the comparison-overlap error is produced; use the filtering example in mypy/plugins/default.py as a reference. Done means non-overlapping comparisons in assert statements no longer emit this error while ordinary comparisons retain their existing behavior.