fix: always delete all backups from an owned test instance

An instance can only be deleted once all backups on the instance have been
deleted. The integration test environment should therefore first delete all
backups on an owned instance before trying to delete the instance itself.

Fixes #542

google-cloud-spanner/src/test/java/com/google/cloud/spanner/IntegrationTestEnv.java

@@ -165,6 +165,18 @@ private void cleanUpInstance() {
       if (isOwnedInstance) {
         // Delete the instance, which implicitly drops all databases in it.
         try {
+          // Backups must be explicitly deleted before the instance may be deleted.
+          logger.log(
+              Level.FINE, "Deleting backups on test instance {0}", testHelper.getInstanceId());
+          for (Backup backup :
+              testHelper
+                  .getClient()
+                  .getDatabaseAdminClient()
+                  .listBackups(testHelper.getInstanceId().getInstance())
+                  .iterateAll()) {
+            logger.log(Level.FINE, "Deleting backup {0}", backup.getId());
+            backup.delete();
+          }
           logger.log(Level.FINE, "Deleting test instance {0}", testHelper.getInstanceId());
           instanceAdminClient.deleteInstance(testHelper.getInstanceId().getInstance());
           logger.log(Level.INFO, "Deleted test instance {0}", testHelper.getInstanceId());