Force usage of assert() condition when NDEBUG is defined

Reuse the BT2 approach to force the usage of the assertion condition
even when assert() are removed by the NDEBUG define.

See `BT_USE_EXPR()` macro and documentation in Babeltrace commit[0]:
  commit 1778c2a4134647150b199b2b57130817144446b0
  Author: Philippe Proulx <eeppeliteloop@gmail.com>
  Date:   Tue Apr 21 11:15:42 2020 -0400
  lib: assign a unique ID to each pre/postcond. and report it on failure

0: https://github.com/efficios/babeltrace/commit/1778c2a4134647150b199b2b57130817144446b0

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3844b6ae7e95952d90033898397ac936540b785c