Introduce a new option DEBUG=attach to run a test against existing
already running vpp. A new target 'make test-start-gdb' will spawn VPP
in gdb for this purpose. Customization options explained in test-help.
Type: improvement
Change-Id: Ia160a85b33da3b2df292d44bb95729af9dd9da96
Signed-off-by: Klement Sekera <ksekera@cisco.com>