Add e2e tests for build/diff kustomization

cmd/flux/build_kustomization.go

cmd/flux/build_kustomization_test.go

cmd/flux/diff_kustomization.go

cmd/flux/diff_kustomization_test.go

cmd/flux/main_test.go

...stdata/build-kustomization/delete-service/deployment.yaml

...flux/testdata/build-kustomization/delete-service/hpa.yaml

...ata/build-kustomization/delete-service/kustomization.yaml

...x/testdata/build-kustomization/podinfo-kustomization.yaml

cmd/flux/testdata/build-kustomization/podinfo-result.yaml

cmd/flux/testdata/build-kustomization/podinfo-source.yaml

...a/build-kustomization/podinfo-without-service-result.yaml

...flux/testdata/build-kustomization/podinfo/deployment.yaml

cmd/flux/testdata/build-kustomization/podinfo/hpa.yaml

...x/testdata/build-kustomization/podinfo/kustomization.yaml

cmd/flux/testdata/build-kustomization/podinfo/service.yaml

...flux/testdata/build-kustomization/podinfo/token.encrypted

cmd/flux/testdata/diff-kustomization/deployment.yaml

...x/testdata/diff-kustomization/diff-with-deployment.golden

...ff-kustomization/diff-with-drifted-key-sops-secret.golden