Implement multi-stage test registry interface

[AlexNPavel]

This implements a few new structs to allow us to read in user test configs and a function to expand the user's config (which may contain references, chains, etc) into a full internal test representation. This contains some duplicate code from TestStepConfiguration as this is essentially the evolution of that struct, but I did not want to trample that code while this new config format is still in a WIP state.

/cc @stevekuznetsov @bbguimaraes

[stevekuznetsov]

This is a good first start!

General comment -- we should make the new type of test just another option, like all of the others we have in the larger TestStepConfiguration -- this will mean we get a lot of validation for free. Once we do that, we should ensure we have new unit tests for those validations.

Also, we'd like to validate that no two steps in the resulting TestFlow have the same name, so we should add a validation function there and unit-test that as well.

The implementation of the Resolver interface should also have a set of unit tests.

[stevekuznetsov]

This is looking really good! Let's add unit tests for Resolve, add in the call to use the resolver in CI Operator and I think we are close to done!

The unit test could look something like:

func TestResolve(t *testing.T) {
    var testCases = []struct{
        name string
        input api.MultiStageTestConfiguration
        expected types.TestFlow
        expectedError bool
    } {
        name: ""
        // ...
    }

    for _, testCase := range testCases {
        t.Run(testCase.name, func(t *testing.T) {
            actual, err := NewResolver().Resolve(testCase.input)
            // check things
        }
    }
}

[stevekuznetsov]

I still think we need a set of unit tests for the Resolve() method like I mentioned in the last review. Otherwise, this looks good!

[stevekuznetsov]

Other than the typo that Petr noticed, this is ready to merge!

[stevekuznetsov]

/lgtm
/approve

[openshift-ci-robot]

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