signal: Add test for SIG_IGN functionality.

signal/example_sigint_ign.py

@@ -0,0 +1,10 @@
+import time
+from signal import *
+
+
+print("org signal() val:", signal(SIGINT, SIG_IGN))
+print("read back signal() val:", signal(SIGINT, SIG_IGN))
+
+print("You can't break me - use SIGTERM")
+while 1:
+    time.sleep(1)