BTS_Tests: properly release DCCH in f_TC_acch_overpower()

Currently the TC_acch_overpower_* testcases are all passing without
any sporadic failures.  However, most of them start failing due to
a DTE (some RSLEM related race condition) if I apply a patch [1] to
trxcon removing its internal TDMA clock module.

I don't know why this happens, but releasing the DCCH after executing
all testcase steps in f_TC_acch_overpower() makes that DTE go away.

Change-Id: I658e78ad8d4dc86403d22b5380ddd9a140f8c71c
Related: [1] osmocom-bb.git Ic8a5b6277c6b16392026e0557376257d71c9d230
Related: OS#5500
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index d03843d..8d02c40 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -8656,6 +8656,10 @@
 
 		setverdict(pass);
 	}
+
+	f_rsl_chan_deact();
+	f_L1CTL_DM_REL_REQ(L1CTL, g_chan_nr);
+	f_rslem_unregister(g_pars.trx_nr, g_chan_nr);
 }
 testcase TC_acch_overpower_rxqual_thresh() runs on test_CT {
 	var ConnHdlrPars pars;