testapp: gen-ust-events: augment captured fields

Adds:
 - negative integer,
 - enums,
 - escaped string,
 - network array,
 - network sequence,
 - long sequence.

This partly bridges the gap between the event used for kernel testing
and the event used for userspace testing.

Bitfield array/sequence are not supported by the userspace tracer.

Change-Id: I027b9acfdcea2643201020c6c7907483cbbb2151
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479