Port Ginkgo tests to pkg/runtime/testenv

[hiddeco]

We are dropping Ginkgo across controllers for testing because it introduced a lot of complexity and a way of writing test no-one till date really liked. As a replacement, testenv should be used to write Go idiomatic tests.

Work like #322 likely has to go first.

An example of such a port for another controller can be found here: fluxcd/image-automation-controller#221

[stefanprodan]

Implemented in #738