derp/derphttp: fix netcheck HTTPS probes

The netcheck client, when no UDP is available, probes distance using
HTTPS.

Several problems:

* It probes using /derp/latency-check.
* But cmd/derper serves the handler at /derp/probe
* Despite the difference, it work by accident until c8f4dfc8c0
  which made netcheck's probe require a 2xx status code.
* in tests, we only use derphttp.Handler, so the cmd/derper-installed
  mux routes aren't preesnt, so there's no probe. That breaks
  tests in airplane mode. netcheck.Client then reports "unexpected
  HTTP status 426" (Upgrade Required)

This makes derp handle both /derp/probe and /derp/latency-check
equivalently, and in both cmd/derper and derphttp.Handler standalone
modes.

I notice this when wgengine/magicsock TestActiveDiscovery was failing
in airplane mode (no wifi). It still doesn't pass, but it gets
further.

Fixes #11989

Change-Id: I45213d4bd137e0f29aac8bd4a9ac92091065113f

cmd/derper/derper.go

@@ -191,7 +191,12 @@ func main() {
 			http.Error(w, "derp server disabled", http.StatusNotFound)
 		}))
 	}
-	mux.HandleFunc("/derp/probe", probeHandler)
+
+	// These two endpoints are the same. Different versions of the clients
+	// have assumes different paths over time so we support both.
+	mux.HandleFunc("/derp/probe", derphttp.ProbeHandler)
+	mux.HandleFunc("/derp/latency-check", derphttp.ProbeHandler)
+
 	go refreshBootstrapDNSLoop()
 	mux.HandleFunc("/bootstrap-dns", tsweb.BrowserHeaderHandlerFunc(handleBootstrapDNS))
 	mux.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -370,17 +375,6 @@ func isChallengeChar(c rune) bool {
 		c == '.' || c == '-' || c == '_'
 }
 
-// probeHandler is the endpoint that js/wasm clients hit to measure
-// DERP latency, since they can't do UDP STUN queries.
-func probeHandler(w http.ResponseWriter, r *http.Request) {
-	switch r.Method {
-	case "HEAD", "GET":
-		w.Header().Set("Access-Control-Allow-Origin", "*")
-	default:
-		http.Error(w, "bogus probe method", http.StatusMethodNotAllowed)
-	}
-}
-
 var validProdHostname = regexp.MustCompile(`^derp([^.]*)\.tailscale\.com\.?$`)
 
 func prodAutocertHostPolicy(_ context.Context, host string) error {