tests: ztest: ztress: Disable on qemu_cortex_r5

This commit disables the `testing.ztest.ztress` test on
`qemu_cortex_r5` because it fails at run-time when compiled with
GCC 12.

Revert this commit when the GitHub issue #49494, which tracks this bug,
is fixed.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>