gh-116604: Fix test_gc on free-threaded build (#116662)

The free-threaded GC only does full collections, so it uses a threshold that
is a maximum of a fixed value (default 2000) and proportional to the number of
live objects. If there were many live objects after the previous collection,
then the threshold may be larger than 10,000 causing
`test_indirect_calls_with_gc_disabled` to fail.

This manually sets the threshold to `(1000, 0, 0)` for the test. The `0`
disables the proportional scaling.

Lib/test/support/__init__.py

@@ -797,6 +797,16 @@ def disable_gc():
         if have_gc:
             gc.enable()
 
+@contextlib.contextmanager
+def gc_threshold(*args):
+    import gc
+    old_threshold = gc.get_threshold()
+    gc.set_threshold(*args)
+    try:
+        yield
+    finally:
+        gc.set_threshold(*old_threshold)
+
 
 def python_is_optimized():
     """Find if Python was built with optimizations."""