1.0.37.20: Make stress test for SB-CONCURRENCY:MAILBOX more robust.

1.0.37.20: Make stress test for SB-CONCURRENCY:MAILBOX more robust.

  * The MAILBOX.INTERRUPT-SAFETY.1 test kills threads randomly while
    stress-testing a mailbox--in the test we made sure that at least
    one receiver remains to properly finish the test; however, some of
    the receiver threads were written with a specific upper bound of
    runs. In the unlikely event that only those receivers remain,
    there was a chance that they could not properly finish the test.

  * We rewrite the receivers to do their work until they receive a
    special FIN token as message. After all senders are done, we make
    sure to send enough FIN for all receivers to shut down.

  * Also gather some more information during the test so in case of
    failure we can gain understanding on what went wrong.