avifil32: Skip tests when we couldn't create or open the AVI file

[Juan Lang]

> Nevertheless, I do think the case could happen again (not enough disk space
> for instance).

IMO, cases like this aren't worth worrying about.  They're like out of
memory conditions:  sure, they can happen, but there's not much we can
do about them.  The disk space requirements of the tests aren't large,
so testers should ensure they have a reasonable amount available.
--Juan