Better fix for the issue with dives/test20.xml

As Anton pointed out, the issue with test20.xml is not that it is
inconsistent, but that it uses the simple gas change event that only
tracks the O2 value. One could argue that this is still a problem in the
presence of trimix gases, but still, there is a much better helper
function available for this situation which doesn't do the simplistic
exact match but looks for the closest gas and is therefore much more
tolerant to issues like this.

With this commit we are switching to using that.

Reported-by: Anton Lundin <glance@acc.umu.se>
Fix-suggested-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>