net: l2: ieee802154: mgmt: improve association procedure spec compliance

This change introduces test coverage for association request and
response. Based on this coverage, several closely related issues were
found in the association process which cannot be split into separate
changes without breaking the build.

Most notably did the associate and disassociate net_mgmt commands send
already encoded IEEE 802.15.4 MPDUs to L3 rather than L2. L3 treated
them as payload and made L2 wrap them with another LL header/footer
which produced invalid packets.

The tests also enforce better aligment of the association process with
the IEEE 802.15.4-2020 standard:

* Association requests now ask for ACK as required by the standard. The
  fake driver was enhanced to produce ACK packages when requested.
* macPanId and macCoordinator* MAC PIB attributes are set in the right
  order for improved filtering of association responses.
* The coordinator may decide not to assign a short address to the end
  device even when associated. This is now supported.
* The coordinator may or may not use a short address. Coordinators
  choosing not to support short addresses are now supported.
* Updating the association will now remove any previously added short
  address from the hardware filter.
* The short address may no longer be changed by the user while
  associated to a PAN. Only the coordinator is allowed to allocate short
  addresses.
* Validation of outgoing and incoming association request/response
  packets is improved.

All changes are documented by pointers into the spec.

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