tests: net: l2: ieee802154: active scan shell/mgmnt coverage

This change introduces integration test coverage for the
stack's scan shell command including the underlying net
management and command handling methods.

Later changes will build on this test to ensure that
changes to command handling will not break backwards
compatibility

This is also the reason why a separate test suite and
test file are being introduced already as further tests
will be added later on. We avoid having to move code
and thereby loose history.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>