tbf/test: Add test for DL TBF exhaustion

This test calls gprs_rlcmac_dl_tbf::handle() with varying TLLI and
IMSI until the function fails.

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.ok b/tests/tbf/TbfTest.ok
index 13d1458..cd96caa 100644
--- a/tests/tbf/TbfTest.ok
+++ b/tests/tbf/TbfTest.ok
@@ -1,2 +1,4 @@
 === start test_tbf_delayed_release ===
 === end test_tbf_delayed_release ===
+=== start test_tbf_exhaustion ===
+=== end test_tbf_exhaustion ===