Tests fail when run on IPv6-only host

[gibmat]

Two tests try to download a test file from GitHub, which is only available over IPv4 networks. This causes those tests to fail if they are run on an IPv6-only host. Please consider bundling the test file with this library, or hosting it on a server that is reachable over IPv6.

=== RUN   TestLayerConvertFunc
    estargz_test.go:37: Get "https://github.com/AkihiroSuda/test-oci-archives/releases/download/v20210101/hello-world.tar.gz": dial tcp 140.82.114.4:443: connect: network is unreachable
--- FAIL: TestLayerConvertFunc (0.05s)
FAIL
FAIL	github.com/containerd/stargz-snapshotter/nativeconverter/estargz	0.094s
=== RUN   TestLayerConvertFunc
    zstdchunked_test.go:40: Get "https://github.com/AkihiroSuda/test-oci-archives/releases/download/v20210101/hello-world.tar.gz": dial tcp 140.82.114.4:443: connect: network is unreachable
--- FAIL: TestLayerConvertFunc (0.00s)
FAIL
FAIL	github.com/containerd/stargz-snapshotter/nativeconverter/zstdchunked	0.101s