health: permit Tracker method calls on nil receiver

In prep for tsd.System Tracker plumbing throughout tailscaled,
defensively permit all methods on Tracker to accept a nil receiver
without crashing, lest I screw something up later. (A health tracking
system that itself causes crashes would be no good.) Methods on nil
receivers should not be called, so a future change will also collect
their stacks (and panic during dev/test), but we should at least not
crash in prod.

This also locks that in with a test using reflect to automatically
call all methods on a nil receiver and check they don't crash.

Updates #11874
Updates #4136

Change-Id: I8e955046ebf370ec8af0c1fb63e5123e6282a9d3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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