[red-knot] Improve the style of various mdtests

[AlexWaygood]

We have a lot of mdtests in red-knot that have this pattern in them to create instance types:

def bool_instance() -> bool:
    return True

x = bool_instance()

def int_instance() -> int:
    return 42

y = int_instance()

# do some reveal_type calls or whatever in the test now with `x` and `y`

Now that #14802 has landed, however, we don't need nearly as much boilerplate in our tests. Instead, we can do something much more elegant:

def f(x: bool, y: int):
    ...
    # do some reveal_type calls or whatever with `x` and `y`

We should go through all the Python snippets in https://github.com/astral-sh/ruff/tree/main/crates/red_knot_python_semantic/resources/mdtest to reduce the boilerplate in our mdtests.

[InSyncWithFoo]

I'll take this. PR coming tomorrow.