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
15fc6cd966