Rather than building a new suggested exit node set every time, compute
it once on first use. Currently, syspolicy ensures that values do not
change without a restart anyway.
Since the set is being constructed in a separate func now, the test code
that manipulates syspolicy can live there, and the TestSuggestExitNode
can now run in parallel with other tests because it does not have global
dependencies.
Updates tailscale/corp#19681
Change-Id: Ic4bb40ccc91b671f9e542bd5ba9c96f942081515
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
cf9f507d47