Add unit tests for export

cmd/flux/export.go

cmd/flux/export_test.go

cmd/flux/testdata/export/alert.yaml

cmd/flux/testdata/export/bucket.yaml

cmd/flux/testdata/export/git-repo.yaml

cmd/flux/testdata/export/helm-release.yaml

cmd/flux/testdata/export/helm-repo.yaml

cmd/flux/testdata/export/image-policy.yaml

cmd/flux/testdata/export/image-repo.yaml

cmd/flux/testdata/export/image-update.yaml

cmd/flux/testdata/export/ks.yaml

cmd/flux/testdata/export/objects.yaml

cmd/flux/testdata/export/provider.yaml

cmd/flux/testdata/export/receiver.yaml