net/http/httptest: panic on non-3 digit (XXX) status code in Recorder.WriteHeader

This change conforms Recorder with net/http servers, to panic
when a handler writes a non-3 digit XXX status code.

Fixes #45353

Change-Id: Id5ed4af652e8c150ae86bf50402b800d935e2203
Reviewed-on: https://go-review.googlesource.com/c/go/+/308950


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>