Runtime-checkable protocol tests: Use @runtime_checkable, not @runtime

[AlexWaygood]

The original name for this decorator was @runtime, and so that's the name that's currently used in most of the tests for this decorator in typing_extensions. However, the name was changed to @runtime_checkable before the decorator was added to the typing module, and so the old @runtime name doesn't appear anywhere in the docs for the typing module. The @runtime name exists in typing_extensions only for backwards compatibility.

Let's change the usages in our test suite to use the more modern name, so that it's less confusing and more consistent with typing.