unit/test-gatt: Add test for unknown request

This send an unknown request (0xbf) to test if a response is sent since
the spec mandates the server to always respond to all requests it
receives:

BLUETOOTH SPECIFICATION Version 5.0 | Vol 3, Part F page 2173

  'A client may send attribute protocol requests to a server, and the
  server shall respond to all requests that it receives.'
1 file changed