Maybe missing document of deps for "make test" to work.

[yangchi]

Not sure if this is due to my dev environment or something else, but I think there are a couple more deps or steps one need to do to make "make test" work, beyond the pre-requisites currently mentioned in README.

In my case I also needed to install sigs.k8s.io/controller-runtime/tools/setup-envtest and then have KUBEBUILDER_ASSETS point to kubebuilder-envtest to make it work, for example.

I also don't know what is the expected coverage when running the test. I'm seeing the following:

        github.com/monzo/egress-operator                coverage: 0.0% of statements
        github.com/monzo/egress-operator/api/v1         coverage: 0.0% of statements
ok      github.com/monzo/egress-operator/controllers    18.973s coverage: 68.7% of statements

I think it might be useful to also document this so users can know if they numbers they get from running tests are in line with a correct expectation.