net/netcheck: extract Compare function for region latencies (#20658)

There are two places in the code where we need to compare the
latencies of a netcheck report. In both cases, the comparison wasn’t
well tested.

This PR extracts that logic into a Compare method of the new
RegionLatency type. This type wraps the map of latency measurements
keyed by region ID.

Updates #cleanup

Change-Id: I7f248988973007c2f452283c1b82f84b03068f77

Signed-off-by: Simon Law <sfllaw@tailscale.com>