Fix the testprovider to not always replace

The provider previously triggers replaces on every update, because it replaces on `prefix` but doesn't store it into outputs.  This *also* triggers the new refresh logic to report an update as part of the refresh, but it seems it is indeed just "broken" even outside of refresh, we just weren't testing multiple consecutive updates to these resources previously.

tests/testprovider/random.go

@@ -39,6 +39,10 @@ func init() {
 					TypeSpec:    pschema.TypeSpec{Type: "integer"},
 					Description: "The length of the random string (not including the prefix, if any).",
 				},
+				"prefix": {
+					TypeSpec:    pschema.TypeSpec{Type: "string"},
+					Description: "An optional prefix.",
+				},
 				"result": {
 					TypeSpec:    pschema.TypeSpec{Type: "string"},
 					Description: "A random string.",
@@ -128,6 +132,9 @@ func (p *randomResourceProvider) Create(ctx context.Context, req *rpc.CreateRequ
 		"length": n,
 		"result": prefix + result,
 	})
+	if prefix != "" {
+		outputs["prefix"] = resource.NewStringProperty(prefix)
+	}
 	outputs["result"] = resource.MakeSecret(outputs["result"])
 
 	outputProperties, err := plugin.MarshalProperties(