Add ginkgo Make target

[agrski]

What this PR does / why we need it:
Running tests in the operator module requires ginkgo.
There is no target in the Makefile to install ginkgo and no documentation on this either.
This PR adds the target and uses it when running tests under <ROOT>/operator.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[agrski]

From looking at the workflow for operator/test, I can see it's run in core-builder, the Dockerfile for which has a lot of dependencies.
Running make test locally still fails for me due to missing these dependencies, e.g. kubebuilder.

As such, I suspect adding a ginkgo target isn't actually useful.
Perhaps it's be more useful to have a target that essentially does what the workflow does - run make test in the right container?

[axsaucedo]

Nice one
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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[agrski]

For anyone reading this later: installing ginkgo gets one step on, but be aware that you'll still need kubebuilder and possibly other tools.

The best way to use the make targets locally is to use the seldonio/core-builder image and run make -C operator test in that, as in the operator test workflow.