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

derp/derphttp/derphttp_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"net/netip"
 	"sync"
 	"testing"
@@ -464,3 +465,24 @@ func TestLocalAddrNoMutex(t *testing.T) {
 		t.Errorf("got error %q; want %q", got, want)
 	}
 }
+
+func TestProbe(t *testing.T) {
+	h := Handler(nil)
+
+	tests := []struct {
+		path string
+		want int
+	}{
+		{"/derp/probe", 200},
+		{"/derp/latency-check", 200},
+		{"/derp/sdf", http.StatusUpgradeRequired},
+	}
+
+	for _, tt := range tests {
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, httptest.NewRequest("GET", tt.path, nil))
+		if got := rec.Result().StatusCode; got != tt.want {
+			t.Errorf("for path %q got HTTP status %v; want %v", tt.path, got, tt.want)
+		}
+	}
+}