devicetree: Fix Device tree tests cannot be built with coverage enabled

DT_SPEC related macro tests (ADC_DT_SPEC_GET_BY_NAME, MBOX_DT_SPEC_GET)
should not appear in this test because the configuration does not turn
on any devices

Move DP_SPEC-related tests to api_ext and enable the related device in the
configuration

fix #77205

Signed-off-by: Wenbin Zhang <freey7955@gmail.com>