tests: power_mgmt: Fix CI build failure

The changes to re-organize the power mgmt header files exposed a
build failure on thingy53_nrf5340_cpunet since CONFIG_PM can't be
set on that platform.  We already exclude nrf5340dk_nrf5340_cpunet
so just add thingy53_nrf5340_cpunet to the platform_exclude list.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>