internal/http3: clean up stream I/O methods usages in tests

In CL 796400, I/O methods were added to the internal/http3.stream. As a
follow-up to that CL, modify our tests to stop calling I/O methods from
internal/quic.stream.

Also add internal/http.stream.WriteByte method for symmetry.

For golang/go#70914

Change-Id: I2036589d89ceac97f80e3f34a29cea786a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/797280
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>