Fix @reap_threads usage in test_free_threading.test_json (#158937)

Use the decorator on test methods, not on the whole class.

Previously, test_json ran no tests:

    $ ./python -m test -v test_free_threading.test_json -v
    ...
    Ran 0 tests in 0.000s

    NO TESTS RAN

Add also a check in @reap_threads to detect misuse (on a class).

Lib/test/support/threading_helper.py

diff --git a/Lib/test/support/threading_helper.py b/Lib/test/support/threading_helper.py
index cf87233..31be31a 100644
--- a/Lib/test/support/threading_helper.py
+++ b/Lib/test/support/threading_helper.py
@@ -1,6 +1,7 @@
 import _thread
 import contextlib
 import functools
+import inspect
 import sys
 import threading
 import time
@@ -59,6 +60,11 @@ def reap_threads(func):
     """Use this function when threads are being used.  This will
     ensure that the threads are cleaned up even when the test fails.
     """
+
+    if inspect.isclass(func):
+        # It should be used on each test method, not on the whole class
+        raise ValueError("@reap_threads is not compatible with classes")
+
     @functools.wraps(func)
     def decorator(*args):
         key = threading_setup()

Lib/test/test_free_threading/test_json.py

diff --git a/Lib/test/test_free_threading/test_json.py b/Lib/test/test_free_threading/test_json.py
index 010eb32..7e3adcf 100644
--- a/Lib/test/test_free_threading/test_json.py
+++ b/Lib/test/test_free_threading/test_json.py
@@ -29,12 +29,12 @@ def items(self):
         return self.mapping
 
 
-@threading_helper.reap_threads
 @threading_helper.requires_working_threading()
 class TestJsonEncoding(CTest):
     # Test encoding json with concurrent threads modifying the data cannot
     # corrupt the interpreter
 
+    @threading_helper.reap_threads
     def test_json_mutating_list(self):
         def worker(barrier, data, index):
             barrier.wait()
@@ -48,6 +48,7 @@ def worker(barrier, data, index):
         data = [[], []]
         encode_json_helper(self.json, worker, data)
 
+    @threading_helper.reap_threads
     def test_json_mutating_exact_dict(self):
         def worker(barrier, data, index):
             barrier.wait()
@@ -65,6 +66,7 @@ def worker(barrier, data, index):
         data = [{}, {}]
         encode_json_helper(self.json, worker, data)
 
+    @threading_helper.reap_threads
     def test_json_mutating_mapping(self):
         def worker(barrier, data, index):
             barrier.wait()