🌱 Migrate envtest setup to envtest.Run, add CAPI_DISABLE_TEST_ENV option

[sbueringer]

Signed-off-by: Stefan Büringer [email protected]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5029

[sbueringer]

First version to find out if we have consensus on the implementation. After we got that, I would extend the PR to add a few sentences how to:

• run unit tests with disabled test env
• run unit tests with an existing Kubernetes cluster (ref: USE_EXISTING_CLUSTER)

to: https://cluster-api.sigs.k8s.io/developer/testing.html#running-unit-and-integration-tests

[sbueringer]

And now also fixed some nil pointers I missed before :/. At least locally all tests are successful now.

@vincepri PTAL :) (I also answered to the remaining open conversations above)

[vincepri]

Let's squash commits

[vincepri]

And retitle the PR 🙃

[sbueringer]

@vincepri rebased, ajusted commit message, retitled PR. Feel free to retitle, I tried to express the main points and keep it somewhat concise

P.S. I assumed that we want to keep utilruntime.Must(AddToScheme(scheme.Scheme)) as is

[fabriziopandini]

/lgtm

[vincepri]

/approve

[k8s-ci-robot]

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[sbueringer]

/retest
the usual flake