The c2n handling code was using the Go httptest package's
ResponseRecorder code but that's in a test package which brings in
Go's test certs, etc.
This forks the httptest recorder type into its own package that only
has the recorder and adds a test that we don't re-introduce a
dependency on httptest.
Updates #12614
Change-Id: I3546f49972981e21813ece9064cc2be0b74f4b16
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
