Fix TestAccSpannerInstance_spannerInstanceWithAutoscaling in replaying mode (#13138) (#21530)

[upstream:e443c06b1cf6ae1249174af24ffc810f2646a127]

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

Author: modular-magician

.changelog/13138.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

google/services/spanner/resource_spanner_instance_test.go

@@ -604,6 +604,7 @@ resource "google_spanner_instance" "main" {
 func testAccSpannerInstance_spannerInstanceWithAutoscaling(context map[string]interface{}) string {
 	return acctest.Nprintf(`
 resource "google_spanner_instance" "example" {
+  name         = "tf-test-spanner-instance-%{random_suffix}"
   config       = "regional-us-central1"
   display_name = "Test Spanner Instance"
   autoscaling_config {