tests: test_pipe_api: increase stack size

A recent patch added some new tests here. Unfortunately,
the current stack size value of 512 was too small for Xtensa.
Increase it to 1024.

Change-Id: I16c52b74412cbd7665e774ce3baed260885ddb9b
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>