[Conformance][RFC] Adjust `NoReturn` conformance test to model the spec more closely

The conformance test for [specialtypes_never.py (lines 28-33)](https://github.com/python/typing/blob/main/conformance/tests/specialtypes_never.py#L28-L33) seems stricter than what the spec requires: ``` def func2(x: int) -> int: if x > 0: return x stop() ... # E?: This statement can be marked unreachable return "whatever works" # No type error ``` The [spec's example](https://typing.python.org/en/latest/spec/special-types.html#noreturn) for a similar scenario says that the error "might be not reported by some checkers" but the conformance test uses `# No type error`, requiring all checkers to suppress the error. There's also an inconsistency: lines 32 and 33 are equally unreachable after `stop()`, but the test allows an optional error on one (# E?) while forbidding any error on the other. If this is about reachability, it seems that both should be treated the same? The `NoReturn` recognition part is already tested by `func2` itself. If a checker didn't recognize `stop()` as `NoReturn`, it would flag the function for not returning `int` on all paths. It seems that the lines after `stop()` are testing dead-code error suppression policy, which the spec leaves to the implementation? Suggested change: Would it make sense to remove lines 32-33 to get: ``` def func2(x: int) -> int: if x > 0: return x stop() ``` This still tests that checkers recognize `NoReturn` calls, without mandating a specific error suppression policy. Would appreciate any thoughts on whether this reading is correct.