tests: Use a pipe rather than a socketpair in stream-rw_all test
The test only needs a unidirectional channel, and a pipe is sufficient for that.
This may simplify things when running the test on Hurd; see #3148 (comment 1874198).
Additionally, set O_NONBLOCK
on the pipe since the test seems to
expect that partial writes will succeed and that writes to a full buffer
will fail rather than block.
Signed-off-by: Philip Withnall philip@tecnocode.co.uk
Helps: #3148
Edited by Philip Withnall