test: enable "response stream error" test under HTTP/2 (TLS)

[pi0x]

The response stream error test in test/_tests.ts is skipped for HTTP/2 runs:

// TODO: Investigate writing test for HTTP2/TLS
test.skipIf(opts.http2)("response stream error", async () => {

It is skipped because streaming a response that errors mid-stream has not yet been reproduced reliably over HTTP/2 (TLS) in the test harness. This issue tracks writing a working HTTP/2 variant so the skip can be removed and stream-error behavior is covered on HTTP/2 as well as HTTP/1.

Tracked as part of the v1 stabilization docs/test-hygiene batch (link skips to issues).