chore: add tests

[skip ci]

stackit/internal/services/postgresflexalpha/postgresflex_acc_test.go

@@ -157,6 +157,9 @@ func TestAccInstance(t *testing.T) {
 	updVersion := updNetACL
 	updVersion.Version = "17"
 
+	updRetention := updVersion
+	updRetention.RetentionDays = 40
+
 	testItemID := testutils.ResStr(pfx, "instance", exData.TfName)
 	compareValuesSame := statecheck.CompareValue(compare.ValuesSame())
 	resource.ParallelTest(
@@ -337,6 +340,23 @@ func TestAccInstance(t *testing.T) {
 						),
 					),
 				},
+				// Update retention_days
+				{
+					PreConfig: func() {
+						t.Logf("  ... %s - %s", t.Name(), "update version")
+					},
+					Config: testutils.StringFromTemplateMust(
+						"testdata/instance_template.gompl",
+						updVersion,
+					),
+					Check: resource.ComposeTestCheckFunc(
+						resource.TestCheckResourceAttr(
+							testItemID,
+							"retention_days",
+							strconv.Itoa(int(updVersion.RetentionDays)),
+						),
+					),
+				},
 				// Import test
 				// test instance imports
 				{