Previously the 'wait' loop would run for ~1024 seconds, at which point
we'd reach the file descriptor limit due to the leak in 'queue-empty?'.
* gnu/tests/mail.scm (run-opensmtpd-test)[test]("mail arrived"): In
'queue-empty?', close PIPE to avoid file descriptor leak. In 'wait'
loop, arrange to run at most 20 times.
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