Add tests for source oci command

cmd/flux/create_source_oci_test.go

cmd/flux/image_test.go

cmd/flux/main_test.go

cmd/flux/source_oci_test.go

cmd/flux/testdata/oci/create_source_oci.golden

cmd/flux/testdata/oci/delete_oci.golden

cmd/flux/testdata/oci/export.golden

cmd/flux/testdata/oci/export_with_secret.golden

cmd/flux/testdata/oci/get_oci.golden

cmd/flux/testdata/oci/reconcile_oci.golden

cmd/flux/testdata/oci/resume_oci.golden

cmd/flux/testdata/oci/suspend_oci.golden