tests/dd: Do not use the OS provided dd utility on FIFOs #5453
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On *BSD and macOS, the system provided dd utility opens up the output file for both reading and writing. This means that the open/write to the FIFO does not block, and almost instantly completes. The system dd then exits, leaving nothing left to be read by the time the coreutils-rs dd tries to open/read the FIFO.
Avoid this problem by just writing to the FIFO from the test case itself, rather than relying on the system provide dd.