Handle Immediate assignment reject

When RACH is received, PCU will generate the Immediate assignment reject
message if no resources are present. The encoding is done based on section
9.1.20 of 44.018 version 11.7.0 Release 11. This patch also includes the
test case to validate the generated Immediate assignment reject message.

This patch is integration tested on Osmo-trx setup with Ettus B210 board
and LG F70 MS with some simulation code changes in Osmo-pcu.

Change-Id: I3d33e2b9746fa4f338fad0e6b63b1c5f07de6f9b
diff --git a/tests/tbf/TbfTest.ok b/tests/tbf/TbfTest.ok
index 2978d6d..f921dfc 100644
--- a/tests/tbf/TbfTest.ok
+++ b/tests/tbf/TbfTest.ok
@@ -68,3 +68,7 @@
 === end test_tbf_li_decoding ===
 === start test_tbf_epdan_out_of_rx_window ===
 === end test_tbf_epdan_out_of_rx_window ===
+=== start test_immediate_assign_rej_multi_block ===
+=== end test_immediate_assign_rej_multi_block ===
+=== start test_immediate_assign_rej_single_block ===
+=== end test_immediate_assign_rej_single_block ===