unit: Make use of tester_monitor to print input/output PDUs

unit: Make use of tester_monitor to print input/output PDUs

tester_monitor will forward the data to btmon when -m/--monitor is
enabled which will attempt to decode the PDUs:

= test-gatt: /robustness/unkown-command - init                                                                                                                        11:44:53.464325
= test-gatt: /robustness/unkown-command - setup
= test-gatt: /robustness/unkown-command - setup complete
= test-gatt: /robustness/unkown-command - run
< test-gatt: User Data TX
      ATT: Exchange MTU Request (0x02) len 2
        Client RX MTU: 23
> test-gatt: User Data RX
      ATT: Exchange MTU Response (0x03) len 2
        Server RX MTU: 512
< test-gatt: User Data TX
      ATT: Unknown (0xff) len 1
        00                                               .
= test-gatt: /robustness/unkown-command - test passed
= test-gatt: /robustness/unkown-command - teardown
= test-gatt: /robustness/unkown-command - teardown complete
= test-gatt: /robustness/unkown-command - done

Change-Id: I44bd1f913f02a953301dc1e0a4b87a448219e7f0
Signed-off-by: himanshu <h.himanshu@samsung.com>