avoid uncompressed duplicates in testdata to make module smaller

[mvdan]

I was investigating what the largest dependencies were in a project by measuring how large the module zips were, and yours stood out at about thirty megabytes. This isn't terrible per se, but it's still surprisingly large, and still slows down some builds depending on the internet speed.

I see that the testdata directory contains some files in both the compressed and uncompressed forms. Have you thought about only keeping the compressed forms?