Set osmo-bts pcu-socket config to point to run dir

Otherwise osmo-bts stores the socket file in /tmp. If an earlier
instance doesn't finish cleanly, the file is left there and a new
instance will fail to start because it cannot create a new socket file
in the same place.

Change-Id: I5a1da23c45a4ac496fe765e0d78c52dae3e7808b
diff --git a/src/osmo_gsm_tester/bts_osmotrx.py b/src/osmo_gsm_tester/bts_osmotrx.py
index 585647a..d8675cb 100644
--- a/src/osmo_gsm_tester/bts_osmotrx.py
+++ b/src/osmo_gsm_tester/bts_osmotrx.py
@@ -95,8 +95,13 @@
 
         values = dict(osmo_bts_trx=config.get_defaults('osmo_bts_trx'))
         config.overlay(values, self.suite_run.config())
-        config.overlay(values, dict(osmo_bts_trx=dict(oml_remote_ip=self.bsc.addr())))
-        config.overlay(values, dict(osmo_bts_trx=self.conf))
+        config.overlay(values, {
+                        'osmo_bts_trx': {
+                            'oml_remote_ip': self.bsc.addr(),
+                            'pcu_socket_path': os.path.join(str(self.run_dir), 'pcu_bts')
+                        }
+        })
+        config.overlay(values, { 'osmo_bts_trx': self.conf })
 
         self.dbg('OSMO-BTS-TRX CONFIG:\n' + pprint.pformat(values))