tests: lib: devicetree devices: only run on native_sim

The test instantiate few vnd,i2c devices so it fails with boards with
devices that select I2C, as that causes i2c_test.c to be included with
the build that tries to redefine the same devices as well.

This is a library test anyway, so restricting to native_sim should be
good enough for it anyway.

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