tests: devicetree: Add a test test_child_nodes_number

The test uses DT_CHILD_NUM and DT_INST_CHILD_NUM to get the number of
child nodes of a given node from generated macro.
The test uses DT_CHILD_NUM_STATUS_OKAY and DT_INST_CHILD_NUM_STATUS_OKAY
to get the number of child nodes of a given node which children's status
are "okay" from generated macro.

Signed-off-by: Swift Tian <swift.tian@ambiq.com>