tests: only provide template values when used

As otherwise the `.golden` values can not be automatically updated using
`-update` as documented in `CONTRIBUTING.md`.

Also ensure we do not use `defer` but rather `t.Cleanup` in tests, as
this will always be called even if e.g. `t.Fatal` absruptly stops the
test.

Signed-off-by: Hidde Beydals <hello@hidde.co>

cmd/flux/build_kustomization_test.go

@@ -171,8 +171,7 @@ spec:
 	if err != nil {
 		t.Fatal(err)
 	}
-
-	defer os.Remove("./testdata/build-kustomization/podinfo.yaml")
+	t.Cleanup(func() { _ = os.Remove("./testdata/build-kustomization/podinfo.yaml") })
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {