[ty] Sort collected diagnostics before snapshotting them in mdtest

[AlexWaygood]

Summary

ty's diagnostics are always sorted into a stable order when ty is executed on the command line. However, this sorting is not applied when the diagnostics are collected by mdtests, which can result in them being snapshotted in our tests in a different order than they would appear to users. That's quite confusing.

This PR fixes that by sorting the diagnostics collected by mdtest in the same way that we sort them when ty is actually executed on the command line.

Test Plan

• cargo test -p ty_python_semantic
• cargo test -p ty_project

[dhruvmanila]

Thank you! I noticed this a couple of times and I was going to open an issue for it.

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[MichaReiser]

Nice, thank you

[MichaReiser]

Nit: I sort of like how TextRange solved this. It has a ordering method. It could help reduce some complexity (no need for an extra struct, less lifetimes)

pub fn ordering(&self, db: &dyn Db, other: &Self) -> Ordering {
	if let (Some(span1), Some(span2)) = (
      self.primary_span(),
      other.primary_span(),
  ) {
      let order = span1
          .file()
          .path(self.db)
          .as_str()
          .cmp(span2.file().path(self.db).as_str());
      if order.is_ne() {
          return order;
      }

      if let (Some(range1), Some(range2)) = (span1.range(), span2.range()) {
          let order = range1.start().cmp(&range2.start());
          if order.is_ne() {
              return order;
          }
      }
  }
  // Reverse so that, e.g., Fatal sorts before Info.
  let order = self
      .severity()
      .cmp(&other.severity())
      .reverse();
  if order.is_ne() {
      return order;
  }
  self.id().cmp(&other.id())
}

[AlexWaygood]

I realised I could simplify it to a single lifetime (pushed the change). I weakly prefer encapsulating the sorting logic into a struct that represents the key (the way I have it now), but no strong opinion; happy to revisit this in a followup if others agree!

[BurntSushi]

I don't feel too strongly personally. I'm fine with either approach given that these aren't semver boundaries.