Fix: test_bitfield: buffer overrun accesses

This issue is due to a type mismatch between union member vs type passed
as parameter to the macros as found by Coverity:

** CID 1401409:  Memory - corruptions  (OVERRUN)
/tests/lib/test_bitfield.c: 450 in run_test_signed_write()

Issue introduced by commit 0675eb8c07 "Extend test_bitfield coverage"

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I14c9d69011b369e5dc8be4305ac8bf32cc86d3dd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1321
CI-Build: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tested-by: jenkins
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>