tests: Avoid sleep when causing a write error

Cause a write error using a fifo and wait, rather than needing a sleep
to wait for something to terminate. This is faster and avoids a
potential test fa

tests: Avoid sleep when causing a write error

Cause a write error using a fifo and wait, rather than needing a sleep
to wait for something to terminate. This is faster and avoids a
potential test failure on a heavily loaded system.

Using /dev/full would be simpler, but it is not portable enough (it
tends not to be available in jails, even). Starting programs with stdout
not open or only open for read/execute/search may have unexpected side
effects.

Reviewed by: ngie, des
Differential Revision: https://reviews.freebsd.org/D57213

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