Bluetooth: tester: Clear adv_buf after sending GAP_EV_DEVICE_FOUND

Avoid sending duplicate GAP_EV_DEVICE_FOUND because adv_buf->len was
not cleared.

With this change, the following tests become more stable in a noisy
environment:
- GAP/BROB/OBSV/BV-01-C
- GAP/BROB/OBSV/BV-02-C
- GAP/BROB/OBSV/BV-05-C
- GAP/BROB/OBSV/BV-06-C
- GAP/DISC/RPA/BV-01-C

Co-authored-by: Konstantinos Sotiropoulos
                konstantinos.sotiropoulos@nordicsemi.no

Signed-off-by: Ryan Chu <ryan.chu@nordicsemi.no>