gh-144766: Fix flaky test_trampoline_works_with_forks

The fork child ran full Python finalization instead of calling
os._exit(0), which is fragile when perf trampoline support is active
(unmapping executable memory and unregistering code watchers during
finalization can crash intermittently). The newer test added in the
same file (test_trampoline_works_after_fork_with_many_code_objects)
already uses os._exit(0) for this reason.

Also fix the parent's wait status handling: os.waitpid returns a raw
wait status, not an exit code. Use os.WEXITSTATUS to extract the
actual exit code, and check os.WIFSIGNALED for signal deaths.

<claude>

Author: yonatan-genai

Lib/test/test_perf_profiler.py

@@ -118,9 +118,12 @@ def foo():
                     if pid == 0:
                         print(os.getpid())
                         baz_fork()
+                        os._exit(0)
                     else:
                         _, status = os.waitpid(-1, 0)
-                        sys.exit(status)
+                        if os.WIFSIGNALED(status):
+                            sys.exit(1)
+                        sys.exit(os.WEXITSTATUS(status))
 
                 def bar():
                     foo()