Now that we have policytest and the policyclient.Client interface, we
can de-global-ify many of the tests, letting them run concurrently
with each other, and just removing global variable complexity.
This does ~half of the LocalBackend ones.
Updates #16998
Change-Id: Iece754e1ef4e49744ccd967fa83629d0dca6f66a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>