This is a test

[toranj] Child Supervision test (#2951)

This commit adds a new test-case under `toranj` for Child Supervision
feature.

This test uses MAC whitelisting to emulate the situation where a child
is removed from parent's child table while the child continues to stay
attached to the parent (since data polls from child are acked at radio
platform layer). Specifically the test verifies that once supervision
check is enabled on the child, the child detects that it is no longer
in the parent's table and tries to re-attach. The test verifies the
behavior of both the parent and the child when supervision is enabled.
It verifies that the parent is periodically sending supervision
messages to the child and that the child is monitoring the messages
from parent. This test also indirectly verify the child timeout
behavior on a parent.