BSC_Tests: fix TC_mgwpool_keepalive_down_not_used

Current osmo-bsc master does not support the MGW keepalive yet, so
f_vty_mgw_cfg_keepalive() fails.  This leaves osmo-bsc in unclean
state and makes all LCLS testcases fail.  The problem is that before
calling it we also call f_vty_mgw_enable() and f_vty_mgw_block(),
but not calling their counterparts.

Rearrange the testcase to call f_vty_mgw_cfg_keepalive() first,
so that we fail early before calling f_vty_mgw_{enable,block}().

Change-Id: I6a94c441fe80a92c237c3c4a5481f2dac3376e35
Fixes: bd59842b6 "bsc: Introduce test TC_mgwpool_keepalive"
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index ba0858c..a9cfa03 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -12307,9 +12307,9 @@
 	f_init(1, true, nr_mgw := 2);
 	f_sleep(1.0);
 
+	f_vty_mgw_cfg_keepalive(0, req_intval := 1, req_endp := "null", rx_timeout := 3);
 	f_vty_mgw_enable(1);
 	f_vty_mgw_block(1, false);
-	f_vty_mgw_cfg_keepalive(0, req_intval := 1, req_endp := "null", rx_timeout := 3);
 
 	vc_conn1 := f_start_handler_create(pars1);
 	vc_conn2 := f_start_handler_create(pars2);