Add tests for zstd content-encoding

[nidhijaju]

This PR adds WPTs for decompressing different types of zstd-encoded content: simple, big, invalid, and an incompatibly large window size. This also reorganizes the tests so they are separated by compression scheme.

[ricea]

lgtm, nice.

[nidhijaju]

@annevk Could you please take a look to see if these WPTs look reasonable? Thank you!

[annevk]

This looks like reasonable zstd coverage. Although perhaps there should be more payloads? Does this also test a variety of invalid payloads and ensure the error handling is correct?

The other thing that is ideally tested is Content-Encoding handling. When it contains multiple values, strange whitespace characters, etc. That can be separate from this PR however.

[nidhijaju]

Does this also test a variety of invalid payloads and ensure the error handling is correct?

I added some tests for invalid payloads similar to the ones for gzip. Please take a look.

[annevk]

Thanks!

I think at some point we might want more exhaustive zstd coverage. Or is there another test suite elsewhere that implementations use?

I also still think we need more Content-Encoding coverage, but it doesn't have to all be in this PR.

[nidhijaju]

Thank you for the review!

is there another test suite elsewhere that implementations use?

We have a set of unit tests in Chromium that we use to test the zstd decompression. Some of them can potentially be converted to WPTs to increase coverage in the future.