fix diff on currency_code in google_billing_budget (#4405) (#8266)

* fix diff on currency_code

* add a test for leaving currency_code blank

* Update resource_billing_budget_test.go

* Update resource_billing_budget_test.go

Signed-off-by: Modular Magician <magic-modules@google.com>

.changelog/4405.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+billing: fixed perma-diff on currency_code in `google_billing_budget`
+```

google/resource_billing_budget.go

@@ -66,6 +66,7 @@ billing account. The currencyCode is provided on output.`,
 								Schema: map[string]*schema.Schema{
 									"currency_code": {
 										Type:        schema.TypeString,
+										Computed:    true,
 										Optional:    true,
 										Description: `The 3-letter currency code defined in ISO 4217.`,
 									},

google/resource_billing_budget_test.go

@@ -0,0 +1,65 @@
+package google
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
+)
+
+func TestAccBillingBudget_billingBudgetCurrencycode(t *testing.T) {
+	t.Parallel()
+
+	context := map[string]interface{}{
+		"billing_acct":  getTestBillingAccountFromEnv(t),
+		"random_suffix": randString(t, 10),
+	}
+
+	vcrTest(t, resource.TestCase{
+		PreCheck:  func() { testAccPreCheck(t) },
+		Providers: testAccProviders,
+		ExternalProviders: map[string]resource.ExternalProvider{
+			"random": {},
+		},
+		CheckDestroy: testAccCheckBillingBudgetDestroyProducer(t),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccBillingBudget_billingBudgetCurrencycode(context),
+				Check:  resource.ComposeTestCheckFunc(),
+			},
+		},
+	})
+}
+
+func testAccBillingBudget_billingBudgetCurrencycode(context map[string]interface{}) string {
+	return Nprintf(`
+data "google_billing_account" "account" {
+  billing_account = "%{billing_acct}"
+}
+
+data "google_project" "project" {
+}
+
+resource "google_billing_budget" "budget" {
+  billing_account = data.google_billing_account.account.id
+  display_name    = "Example Billing Budget%{random_suffix}"
+
+  budget_filter {
+    projects = ["projects/${data.google_project.project.number}"]
+  }
+
+  amount {
+    specified_amount {
+      units         = "100000"
+    }
+  }
+
+  threshold_rules {
+    threshold_percent = 1.0
+  }
+  threshold_rules {
+    threshold_percent = 1.0
+    spend_basis       = "FORECASTED_SPEND"
+  }
+}
+`, context)
+}