Add run-fail to semver-check for docs

[Noratrieb]

I encountered this missing feature in #10276 and therefore added it here in this separate PR.

If the breaking change does not involve a compilation error but a change in runtime behaviour, you can add run-fail to the codeblock. The "before" code must return exit code 0, and the "after" code must be nonzero (like a panic).

Example case that I tested (ignore the trailing dot, it's for github markdown to not hate me)

```rust,ignore,run-fail
// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo() {}
///////////////////////////////////////////////////////////
// After
pub fn foo() {
    panic!("hey!");
}
///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    updated_crate::foo();
}
```.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[ehuss]

Thanks!

@bors r+

[bors]

📌 Commit 6865218 has been approved by ehuss

[bors]

⌛ Testing commit 6865218 with merge d43a507...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing d43a507 to master...