tests: force stack canaries off for two user tests

Stack canaries require that the z_libc_partition be added to
the memory domain, otherwise user thread access to the
stack canary value will result in an MPU/MMU fault.

These tests define their own domains to test specific userspace
features. Adding another partition to them would be invasive,
would potentially break some platforms with a limited number
of MPU regions, and these tests are not designed to validate
stack canaries anyway, we have other tests for that.

Fixes: #17595

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