cmd/containerboot: use synctest in TestRefreshAdvertiseServices

This test was the repo's slowest at 60 seconds of wall time, all of it
spent sleeping: three of its subtests reach the unconditional 20 second
failover wait in kube/services.EnsureServicesAdvertised, despite using
a pure in-memory FakeLocalClient with no real control or I/O.

Run each subtest in a testing/synctest bubble so the wait elapses on
the fake clock instead. The test now completes in milliseconds.

Fixes #20792

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3442815f7efcf6de740f893197ee0461ab049bb2