tests: devicetree: device: avoid multiple declarations

The test defines driver instances for the devices in the tree, but
there's now a dummy driver for vnd,i2c that would also attempt to
define the same device on any platform that defaults CONFIG_I2C=y,
resulting in multiply-defined symbol errors.

native_posix does not default CONFIG_I2C=y, and because
CONFIG_I2C_TEST is hidden there's no way to turn it on or off at the
driver level, so disable the subsystem to mask the driver, leaving the
test code as the only place the devices are defined.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>