14199: Fix deadlock in test suite.

14199: Fix deadlock in test suite.

If a KeepClient has been created using an API server address that is
no longer reachable, calling keepclient.RefreshServiceDiscovery() puts
the poll() goroutine into an endless retry loop, and a second call
never returns because ent.clear is never ready to receive.

Work around this in the pull worker tests by only refreshing services
from the API server actually being used, not on additional ones
referenced by previous test cases.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>