Fix: test_bitfield: extend coverage: 0-len signed write/read

Fix: test_bitfield: extend coverage: 0-len signed write/read

By convention, a 0-len bitfield write is a no-op, and a 0-len read
sets the value of the output to 0.

So we can "encode" the value 0 over a length of 0 bit. Cover this
in the test-cases for signed types. It is already covered for the
unsigned test-cases.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib6038b98167ef2cfa8c504d7ac29498e71827b38
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1341
Tested-by: jenkins