Test fix: passing bool argument to va_start is undefined

Test fix: passing bool argument to va_start is undefined

clang warns that "passing an object that undergoes default argument
promotion to 'va_start' has undefined behaviour [-Wvarargs]".

Since va_start's last argument has no known type, the boolean argument
is promoted to 'int', which is not guaranteed to have the same size
as 'bool'.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>