packetgen: Add options --fuzzing, --testing to customize code generation

Generation of packet fuzzers and packet reflection tests are now
disabled by default, use this option to re-activate them.
The fuzzing and test code are still generated with preprocessing
guards PACKET_FUZZING, PACKET_TESTING which you need to define
to use the generated helpers.

Test: m root-canal
Test: m bluetooth_test_gd_unit
Change-Id: Iac6f5c2f24596bf7537b9340707044ba1e71cf27