gh-155146: Do not depend on the exact number of allocations in test_class

Try to fail every one of the first allocations and accept the first one which
fails in the code detaching the instance dictionary from the object, instead
of assuming that this is the first allocation after set_nomemory().

Run the test with the test.support.isolation.runInSubprocess() decorator
instead of executing it as a source string in a subprocess.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_class.py

@@ -2,7 +2,7 @@
 
 import unittest
 from test import support
-from test.support import cpython_only, import_helper, script_helper
+from test.support import cpython_only, import_helper, isolation
 
 testmeths = [
 
@@ -1014,32 +1014,43 @@ class C:
         C.a = X()
 
     @support.nomemtest
+    @isolation.runInSubprocess()
     def test_detach_materialized_dict_no_memory(self):
-        code = """if 1:
-            import test.support
-            import _testcapi
-
-            class A:
-                def __init__(self):
-                    self.a = 1
-                    self.b = 2
+        import _testcapi
+
+        class A:
+            def __init__(self):
+                self.a = 1
+                self.b = 2
+
+        # The failing allocation should be the one which detaches the
+        # dictionary from the object, but other allocations can happen
+        # first, so try to fail every one of the first allocations.
+        raised = False
+        for n in range(20):
             a = A()
             d = a.__dict__
-            with test.support.catch_unraisable_exception() as ex:
-                _testcapi.set_nomemory(0, 1)
-                del a
-                assert ex.unraisable.exc_type is MemoryError
             try:
-                d["a"]
-            except KeyError:
-                pass
-            else:
-                assert False, "KeyError not raised"
-        """
-        rc, out, err = script_helper.assert_python_ok("-c", code)
-        self.assertEqual(rc, 0)
-        self.assertFalse(out, msg=out.decode('utf-8'))
-        self.assertFalse(err, msg=err.decode('utf-8'))
+                with support.catch_unraisable_exception() as ex:
+                    _testcapi.set_nomemory(n, n + 1)
+                    try:
+                        del a
+                    finally:
+                        _testcapi.remove_mem_hooks()
+                    exc_type = ex.unraisable and ex.unraisable.exc_type
+            except MemoryError:
+                # The failing allocation was not in the deallocation code.
+                continue
+            if exc_type is not MemoryError:
+                continue
+            raised = True
+            if "a" not in d:
+                # The dictionary was cleared, as expected.
+                break
+        else:
+            if not raised:
+                self.fail("MemoryError was not raised during deallocation")
+            self.fail("the dictionary was not cleared")
 
 if __name__ == '__main__':
     unittest.main()