Additional gz test coverage

[brian-pane]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	87.26% <ø> (-2.38%)	⬇️
test-x86_64-apple-darwin	85.55% <ø> (-2.18%)	⬇️
test-x86_64-unknown-linux-gnu	87.08% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
libz-rs-sys/src/gz.rs	89.24% <ø> (-4.89%)	⬇️

... and 4 files with indirect coverage changes

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[brian-pane]

Separate from this PR, the code coverage report shows gaps that I can't figure out. For example, here it shows that gzclose_r isn't invoked with a null argument, but that happens here in the test cases.

[folkertdev]

You can run cargo llvm-cov locally on just the test-libz-rs-sys I to hlget more accurate coverage I believe.

Also we're at rustweek this week, so I'll probably only get to reviewing this next week