[red-knot] mdtest should fail if you have a triple-backtick block that is unclosed

[AlexWaygood]

If you have an mdtest like this at the end of the Markdown file, it doesn't fail:

# bytes should be inferred as strings

```py
x = b"42"

reveal_type(x)  # revealed: str

That's because the backticks are never closed, so mdtest doesn't even recognise this as a test snippet. I keep on making this mistake locally and wondering why my tests aren't even running! It would be ideal if the framework could detect this and tell me that my test file is invalid because the backticks were never closed.

[sharkdp]

Yes, please! Ran into this multiple times already.

[InSyncWithFoo]

This seems trivial to fix. A PR should be ready by Saturday.