DL: add test case to show wrong window size

This patch adds a test case test_tbf_update_ws. Which expects a
current bug with DL window size calculation. The test's expectation
is corrected along with the bug fix in a subsequent commit

Related: OS#1808

Change-Id: I4659494c6f93ae89e4cc4ac79fff5fcaf2d23699
diff --git a/tests/tbf/TbfTest.ok b/tests/tbf/TbfTest.ok
index dda72ef..13ad362 100644
--- a/tests/tbf/TbfTest.ok
+++ b/tests/tbf/TbfTest.ok
@@ -62,3 +62,5 @@
 === end test_tbf_egprs_spb_dl ===
 === start test_tbf_puan_urbb_len ===
 === end test_tbf_puan_urbb_len ===
+=== start test_tbf_update_ws ===
+=== end test_tbf_update_ws ===