gh-108303: Create Lib/test/test_dataclasses/ directory (#108978)

Move test_dataclasses.py and its "dataclass_*.py" modules into the
new Lib/test/test_dataclasses/ subdirectory.

Lib/test/test_dataclasses/dataclass_module_2_str.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+USING_STRINGS = True
+
+# dataclass_module_2.py and dataclass_module_2_str.py are identical
+# except only the latter uses string annotations.
+
+from dataclasses import dataclass, InitVar
+from typing import ClassVar
+
+T_CV2 = ClassVar[int]
+T_CV3 = ClassVar
+
+T_IV2 = InitVar[int]
+T_IV3 = InitVar
+
+@dataclass
+class CV:
+    T_CV4 = ClassVar
+    cv0: ClassVar[int] = 20
+    cv1: ClassVar = 30
+    cv2: T_CV2
+    cv3: T_CV3
+    not_cv4: T_CV4  # When using string annotations, this field is not recognized as a ClassVar.
+
+@dataclass
+class IV:
+    T_IV4 = InitVar
+    iv0: InitVar[int]
+    iv1: InitVar
+    iv2: T_IV2
+    iv3: T_IV3
+    not_iv4: T_IV4  # When using string annotations, this field is not recognized as an InitVar.