test_common: plug subscription manager goroutine leak
test_common: plug subscription manager goroutine leak

The `config` object already has a `subcriptionManager` at this point.
If we overwrite it, then the goroutine started by the original one
will never be stopped on shutdown.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client