Fix stack ztest

While ztest does run in user space we run it with the same stack
restrictions it would have in kernel space. This ensures that any
stack related issues which would be hit in the kernel can be caught
and debugged in user space instead.

This patch is a first pass to limit the stack usage of every ztest
function to 1024 bytes. Subsequent updates can further reduce this.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>