Extend test_bitfield coverage

Extend test_bitfield coverage

test_bitfield was mainly testing various write unit size. Add
variations of read unit size as well.

Previously, the test was only covering input from a 32-bit integer.
Additionally test source and destination of 64-bit.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ic2b7849140e04fe7cca3de576e31bfef8f0a03ae
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1305
Tested-by: jenkins