Panic on test failure

> TestNativeDag/GET_plain_JSON_codec_with_format=json_returns_same_payload_as_format=dag-json_but_with_plain_Content-Type

```
=== RUN TestNativeDag
path_gateway_dag_test.go:356: --- META: {\"group\":\"JSON-CBOR\"}
--- FAIL: TestNativeDag (4.89s)
panic: invalid character '<' looking for beginning of value [recovered]
panic: invalid character '<' looking for beginning of value·
goroutine 355 [running]:
testing.tRunner.func1.2({0x105796b40, 0x14000134030})
/Users/alex/.gvm/gos/go1.24.9/src/testing/testing.go:1734 +0x1ac
testing.tRunner.func1()
/Users/alex/.gvm/gos/go1.24.9/src/testing/testing.go:1737 +0x334
panic({0x105796b40?, 0x14000134030?})
/Users/alex/.gvm/gos/go1.24.9/src/runtime/panic.go:792 +0x124
github.com/ipfs/gateway-conformance/tooling/check.(*CheckIsJSONEqual).Check(0x140005ab080, {0x140004c4000, 0x1e23, 0x2500})
/Users/alex/.gvm/pkgsets/go1.24.9/global/pkg/mod/github.com/ipfs/gateway-conformance@v0.11.1/tooling/check/check.go:363 +0x1a0
github.com/ipfs/gateway-conformance/tooling/test.validateResponse(0x140000036c0?, {0xc8, 0x0, 0x0, {0x14000114ba0, 0x1, 0x1}, {0x105796480, 0x140005ab080}, {0x0, ...}}, ...)
/Users/alex/.gvm/pkgsets/go1.24.9/global/pkg/mod/github.com/ipfs/gateway-conformance@v0.11.1/tooling/test/validate.go:102 +0xeac
github.com/ipfs/gateway-conformance/tooling/test.ExpectBuilder.Validate({0xc8, 0x0, 0x0, {0x14000114ba0, 0x1, 0x1}, {0x105796480, 0x140005ab080}, {0x0, 0x0, ...}}, ...)
/Users/alex/.gvm/pkgsets/go1.24.9/global/pkg/mod/github.com/ipfs/gateway-conformance@v0.11.1/tooling/test/sugar.go:240 +0xd4
github.com/ipfs/gateway-conformance/tooling/test.run.func2(0x140000036c0)
/Users/alex/.gvm/pkgsets/go1.24.9/global/pkg/mod/github.com/ipfs/gateway-conformance@v0.11.1/tooling/test/test.go:100 +0x184
testing.tRunner(0x140000036c0, 0x1400058e020)
/Users/alex/.gvm/gos/go1.24.9/src/testing/testing.go:1792 +0xe4
created by testing.(*T).Run in goroutine 292
/Users/alex/.gvm/gos/go1.24.9/src/testing/testing.go:1851 +0x374
```

The failure is valid, in that I changed something and the test failed, but it's not possible to tell what the failed assertion is here.