[red-knot] Switch to a handwritten parser for mdtest error assertions (#16422)

Author: AlexWaygood

Cargo.lock

@@ -31,6 +31,7 @@ salsa = { workspace = true }
 smallvec = { workspace = true }
 serde = { workspace = true }
 toml = { workspace = true }
+thiserror = { workspace = true }
 
 [lints]
 workspace = true