tests: posix: override qemu_x86_tiny by label

This changes the board override to track the nodelabel rather than the
absolute node path, which happened to have changed in 0edc89c6
breaking the test.

Verified with:

west build -p -b qemu_x86_tiny/atom tests/posix/common -T \
portability.posix.common.userspace

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>