tests: devicetree: device: Exclude test on bbc_microbit

The bbc_microbit boards end up enabling CONFIG_SENSOR because of
CLOCK_CONTROL_NRF_USES_TEMP_SENSOR.  This ends up enabling the I2C
bus which causes CONFIG_I2C_TEST=y.  We end up with a build conflict
betwee the i2c_test.c driver and the test case code.  So the easiest
solution is to just exclude the platforms from this test.

Signed-off-by: Kumar Gala <galak@kernel.org>