BTS: pcu_rr_suspend clean up and release the channel

The testcase failed becaues of an unexpected RSL Error Indication from
the LAPDm system which was in the RSL emulation queue when tearing down
the test environment.
By calling f_rsl_chan_deact() the queue gets flushed until the RSL
deactivate channel is received. It's also more clean to release the
channel.

Fixes: OS#4051
Change-Id: I55827626803ca81b68f905fd0df3126367951f39
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 9371612..3244b48 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -3999,6 +3999,11 @@
 		setverdict(fail, "Timeout waiting for SUSP REQ on PCUIF");
 		}
 	}
+
+	/* release the channel */
+	f_rsl_chan_deact();
+	f_L1CTL_DM_REL_REQ(L1CTL, g_chan_nr);
+	f_rslem_unregister(0, g_chan_nr);
 }
 testcase TC_pcu_rr_suspend() runs on test_CT {
 	var ConnHdlrPars pars;