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.

(cherry picked from commit 14d6e19)

Author: vstinner

Lib/test/test_dataclasses.py renamed to Lib/test/test_dataclasses/__init__.py

@@ -3568,10 +3568,10 @@ class C:
                 self.assertEqual(C(10).x, 10)
 
     def test_classvar_module_level_import(self):
-        from test import dataclass_module_1
-        from test import dataclass_module_1_str
-        from test import dataclass_module_2
-        from test import dataclass_module_2_str
+        from test.test_dataclasses import dataclass_module_1
+        from test.test_dataclasses import dataclass_module_1_str
+        from test.test_dataclasses import dataclass_module_2
+        from test.test_dataclasses import dataclass_module_2_str
 
         for m in (dataclass_module_1, dataclass_module_1_str,
                   dataclass_module_2, dataclass_module_2_str,
@@ -3609,7 +3609,7 @@ def test_classvar_module_level_import(self):
                     self.assertNotIn('not_iv4', c.__dict__)
 
     def test_text_annotations(self):
-        from test import dataclass_textanno
+        from test.test_dataclasses import dataclass_textanno
 
         self.assertEqual(
             get_type_hints(dataclass_textanno.Bar),

Lib/test/dataclass_module_1.py renamed to Lib/test/test_dataclasses/dataclass_module_1.py

@@ -1962,6 +1962,7 @@ TESTSUBDIRS=	ctypes/test \
 		test/test_asyncio \
 		test/test_capi \
 		test/test_cppext \
+		test/test_dataclasses \
 		test/test_email \
 		test/test_email/data \
 		test/test_import \