unit/test-gatt: Add test for unknown request

This send an unknown command (0xff) to test that no response is sent
since the spec mandates the server to not respond to commands:

BLUETOOTH SPECIFICATION Version 5.0 | Vol 3, Part F page 2179

  'If a server receives a command that it does not support, indicated
  by the Command Flag of the PDU set to one, then the server shall
  ignore the Command.'
1 file changed