[red-knot] type inference/checking test framework (#13636)

## Summary

Adds a markdown-based test framework for writing tests of type inference
and type checking. Fixes #11664.

Implements the basic required features. A markdown test file is a suite
of tests, each test can contain one or more Python files, with
optionally specified path/name. The test writes all files to an
in-memory file system, runs red-knot, and matches the resulting
diagnostics against `Type: ` and `Error: ` assertions embedded in the
Python source as comments.

We will want to add features like incremental tests, setting custom
configuration for tests, writing non-Python files, testing syntax
errors, capturing full diagnostic output, etc. There's also plenty of
room for improved UX (colored output?).

## Test Plan

Lots of tests!

Sample of the current output when a test fails:

```
     Running tests/inference.rs (target/debug/deps/inference-7c96590aa84de2a4)

running 1 test
test inference::path_1_resources_inference_numbers_md ... FAILED

failures:

---- inference::path_1_resources_inference_numbers_md stdout ----
inference/numbers.md - Numbers - Floats
  /src/test.py
    line 2: unexpected error: [invalid-assignment] "Object of type `Literal["str"]` is not assignable to `int`"

thread 'inference::path_1_resources_inference_numbers_md' panicked at crates/red_knot_test/src/lib.rs:60:5:
Some tests failed.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    inference::path_1_resources_inference_numbers_md

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.19s

error: test failed, to rerun pass `-p red_knot_test --test inference`
```

---------

Co-authored-by: Micha Reiser <micha@reiser.io>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_trivia/src/comment_ranges.rs

@@ -194,7 +194,7 @@ impl CommentRanges {
     }
 
     /// Returns `true` if a comment is an own-line comment (as opposed to an end-of-line comment).
-    fn is_own_line(offset: TextSize, locator: &Locator) -> bool {
+    pub fn is_own_line(offset: TextSize, locator: &Locator) -> bool {
         let range = TextRange::new(locator.line_start(offset), offset);
         locator.slice(range).chars().all(is_python_whitespace)
     }