Use startBgpServer() rather than doing things individually, so that we
can follow the logic path of how kube-router actually works better. This
allows us to use annotations rather than set stuff manually and allows
us to test more of the code-path of the NRC.
Additionally, this change allows us to actually test some errors better
such as, make sure that startBgpServer() actually throws the error we
expect when only one part of the prepend ASN annotation is present.
Previously, we were not actually testing this code path.
