checked out swift-nio-http2 repo is 63 MiB, 59 MiB of that is because of uncompressed json test fixtures

```
$ du -ms swift-nio-http2/Tests/hpack-test-case/
59 swift-nio-http2/Tests/hpack-test-case/
```

Even gzip compressed it only takes up 5.5 MiB, so we could save a bunch of bandwidth and disk space if we compressed those files and decompressed them only when running the H2 tests.

Start with Tests/hpack-test-case/ and the HTTP/2 tests that consume these JSON fixtures. Check how the fixtures are loaded, then run the H2 tests before and after compressing them and adding test-time decompression. Done means the tests still pass and the checkout no longer stores the fixtures uncompressed.

Written by the indexing model from the issue text.