[mypyc] Fix remaining failing test on free-threaded builds (#19646)

Just skip an in irbuild test if running on a free-threaded build, since
the IR looks different if free threading is enabled.

Only this one test was failing for me on Python 3.14.0rc1.

Author: JukkaL

mypyc/test-data/irbuild-classes.test

@@ -180,7 +180,7 @@ L0:
     o.x = r1; r2 = is_error
     return o
 
-[case testSubclass_toplevel]
+[case testSubclass_withgil_toplevel]
 from typing import TypeVar, Generic
 from mypy_extensions import trait
 T = TypeVar('T')

mypyc/test/test_irbuild.py

@@ -8,7 +8,7 @@
 from mypy.errors import CompileError
 from mypy.test.config import test_temp_dir
 from mypy.test.data import DataDrivenTestCase
-from mypyc.common import TOP_LEVEL_NAME
+from mypyc.common import IS_FREE_THREADED, TOP_LEVEL_NAME
 from mypyc.ir.pprint import format_func
 from mypyc.test.testutil import (
     ICODE_GEN_BUILTINS,
@@ -71,6 +71,9 @@ def run_case(self, testcase: DataDrivenTestCase) -> None:
         if options is None:
             # Skipped test case
             return
+        if "_withgil" in testcase.name and IS_FREE_THREADED:
+            # Test case should only run on a non-free-threaded build.
+            return
         with use_custom_builtins(os.path.join(self.data_prefix, ICODE_GEN_BUILTINS), testcase):
             expected_output = remove_comment_lines(testcase.output)
             expected_output = replace_word_size(expected_output)