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I'm no expert either. The output certainly seems to be deterministic
for a given version of zlib, and I'm not aware of any prior versions of
zlib that produce different compressed output. However, my
understanding is that there is more than one possible compressed
representation of a given uncompressed input, so it's entirely possible
that a past or future version of zlib might produce compressed output
that is different while remaining interoperable. I have no idea whether
the zlib people care specifically about producing identical compressed
output across versions or not. It might be a big deal to them, or they
might have other priorities.
I included the third test because I am guessing that the compressed
output probably won't change very soon, and that if it does, it might be
interesting to know that it changed. If that sounds to you like it
might be more trouble than it's worth, then I think the right thing to
do would be to simply get rid of the third test and keep the first two.