[ty] Restructure/move dataclass tests (#19117)

Before I'm adding even more dataclass-related files, let's organize them
in a separate folder.

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/dataclass_transform.md renamed to crates/ty_python_semantic/resources/mdtest/dataclasses/dataclass_transform.md

@@ -4,7 +4,7 @@ expression: snapshot
 ---
 ---
 mdtest name: dataclasses.md - Dataclasses - `dataclasses.KW_ONLY`
-mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses.md
+mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md
 ---
 
 # Python source files