of: unittest: disable interrupts_property warning

The testcases.dts has purposely bad data which now generates a dtc warning:

drivers/of/unittest-data/testcases.dtb: Warning (interrupts_property): interrupts size is (4), expected multiple of 8 in /testcase-data/testcase-device2

Disable this warning for now. The proper solution is to split the unit
tests into good and bad data.

Signed-off-by: Rob Herring <robh@kernel.org>