osmo-bts-trx: wait some longer before starting osmo-bts-trx

We get sporadic clock skews when running osmo-bts-trx, causing the process to
end and the test to fail. Try to give some seconds for the osmo-trx process to
sort itself out, in the hope to avoid the clock skews.

Related: OS#2325

Change-Id: I82d29358498e7ad9fef28d409808168926e2f876
diff --git a/src/osmo_gsm_tester/bts_osmotrx.py b/src/osmo_gsm_tester/bts_osmotrx.py
index 7d12c85..c8fa594 100644
--- a/src/osmo_gsm_tester/bts_osmotrx.py
+++ b/src/osmo_gsm_tester/bts_osmotrx.py
@@ -64,6 +64,9 @@
         self.log('Waiting for osmo-trx to start up...')
         event_loop.wait(self, self.trx_ready)
         self.proc_trx.log(self.proc_trx.get_stdout_tail(1))
+        self.log('Give some grace time (hopefully to avoid clock skews)')
+        event_loop.sleep(self, 5)
+
         self.launch_process(OsmoBtsTrx.BIN_BTS_TRX, '-r', '1',
                             '-c', os.path.abspath(self.config_file),
                             '-i', self.bsc.addr())