net/http: remove test-only private key from production binaries

The net/http/internal package contains a PEM-encoded private key used in
tests. This key is initialized at init time, which prevents it from
being stripped by the linker in non-test binaries.

Move the certificate and key to a new net/http/internal/testcert
package to ensure it is only included in binaries that reference it.

Fixes #46677.

Change-Id: Ie98bda529169314cc791063e7ce4d99ef99113c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/326771


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>