gh-125900: Clean-up logic around immortalization in free-threading (#125901)

* Remove `@suppress_immortalization` decorator
* Make suppression flag per-thread instead of per-interpreter
* Suppress immortalization in `eval()` to avoid refleaks in three tests
  (test_datetime.test_roundtrip, test_logging.test_config8_ok, and
   test_random.test_after_fork).
* frozenset() is constant, but not a singleton. When run multiple times,
  the test could fail due to constant interning.

Lib/test/test_code.py

@@ -141,9 +141,7 @@ except ImportError:
     ctypes = None
 from test.support import (cpython_only,
                           check_impl_detail, requires_debug_ranges,
-                          gc_collect, Py_GIL_DISABLED,
-                          suppress_immortalization,
-                          skip_if_suppress_immortalization)
+                          gc_collect, Py_GIL_DISABLED)
 from test.support.script_helper import assert_python_ok
 from test.support import threading_helper, import_helper
 from test.support.bytecode_helper import instructions_with_positions
@@ -579,7 +577,6 @@ class CodeConstsTest(unittest.TestCase):
 
     @cpython_only
     @unittest.skipUnless(Py_GIL_DISABLED, "does not intern all constants")
-    @skip_if_suppress_immortalization()
     def test_interned_constants(self):
         # compile separately to avoid compile time de-duping
 
@@ -599,7 +596,6 @@ class CodeConstsTest(unittest.TestCase):
 
 class CodeWeakRefTest(unittest.TestCase):
 
-    @suppress_immortalization()
     def test_basic(self):
         # Create a code object in a clean environment so that we know we have
         # the only reference to it left.
@@ -850,7 +846,6 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             self.assertEqual(GetExtra(f.__code__, FREE_INDEX+100,
                               ctypes.c_voidp(100)), 0)
 
-        @suppress_immortalization()
         def test_free_called(self):
             # Verify that the provided free function gets invoked
             # when the code object is cleaned up.
@@ -878,7 +873,6 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             del f
 
         @threading_helper.requires_working_threading()
-        @suppress_immortalization()
         def test_free_different_thread(self):
             # Freeing a code object on a different thread then
             # where the co_extra was set should be safe.