test(spanner): unflake ITAutogeneratedAdminClientTest

This fixes a timeout in roleWithNoPermissions and grantAndRevokeDatabaseRolePermissions where the spanner DatabaseClient instance was not closed before dropping the role, causing the backend DROP ROLE DDL to block on active session pool connections.

Author: sakthivelmanii

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITAutogeneratedAdminClientTest.java

@@ -166,6 +166,7 @@ public void grantAndRevokeDatabaseRolePermissions() throws Exception {
       assertThat(e.getErrorCode()).isEqualTo(ErrorCode.PERMISSION_DENIED);
       assertThat(e.getMessage()).contains(dbRoleParent);
     }
+    spanner.close();
     // Drop role and table.
     final String dropTableT = "DROP TABLE T";
     final String dropRoleParent = String.format("DROP ROLE %s", dbRoleParent);
@@ -209,6 +210,7 @@ public void roleWithNoPermissions() throws Exception {
       assertThat(e.getErrorCode()).isEqualTo(ErrorCode.PERMISSION_DENIED);
       assertThat(e.getMessage()).contains(dbRoleOrphan);
     }
+    spanner.close();
     // Drop role and table.
     final String dropTableT = "DROP TABLE T";
     final String dropRoleParent = String.format("DROP ROLE %s", dbRoleOrphan);