The '-t' option of GNU gzip allows checking whether a gzip file is valid
without writing the data anywhere. It's relatively straightforward to
support in libdeflate-gzip too, so add support for it.
Resolves https://github.com/ebiggers/libdeflate/issues/125
[EB - updated commit message]
Compression is based on heuristics, so we can't guarantee in every
circumstance that the compression ratio will improve as the compression
level increases. The tests need to be 100% reliable though, so drop
this part of the test.
