IPAM unit tests are very slow

[tomwilkie]

vagrant@ubuntu-14:~/src/github.com/weaveworks/weave/ipam$ go test -tags netgo -cpu 4 -timeout 8m -race -covermode=atomic -coverprofile=coverage.unit.0CMjTDBkTL -coverpkg=github.com/weaveworks/weave/ipam,github.com/weaveworks/weave/common,github.com/weaveworks/weave/common/docker,github.com/weaveworks/weave/ipam/paxos,github.com/weaveworks/weave/ipam/ring,github.com/weaveworks/weave/ipam/space,github.com/weaveworks/weave/net/address,github.com/weaveworks/weave/router -cpuprofile cpu.out .
ok      github.com/weaveworks/weave/ipam    102.159s    coverage: 27.2% of statements in github.com/weaveworks/weave/ipam, github.com/weaveworks/weave/common, github.com/weaveworks/weave/common/docker, github.com/weaveworks/weave/ipam/paxos, github.com/weaveworks/weave/ipam/ring, github.com/weaveworks/weave/ipam/space, github.com/weaveworks/weave/net/address, github.com/weaveworks/weave/router