bpo-40094: mailcap.test() uses waitstatus_to_exitcode() (GH-19287)

mailcap.test() now uses os.waitstatus_to_exitcode() to convert
os.system() exit status into an exit code.

Author: vstinner

Lib/mailcap.py

@@ -251,6 +251,7 @@ def test():
         else:
             print("Executing:", command)
             sts = os.system(command)
+            sts = os.waitstatus_to_exitcode(sts)
             if sts:
                 print("Exit status:", sts)