mgcp: Verify that the force-ptime is written back to the file

When the command was added it was not verified that. Add a very
basic MGCP VTY test and test that the string appears in the config.
diff --git a/openbsc/doc/examples/osmo-bsc_mgcp/mgcp.cfg b/openbsc/doc/examples/osmo-bsc_mgcp/mgcp.cfg
index 7409dd0..791497f 100644
--- a/openbsc/doc/examples/osmo-bsc_mgcp/mgcp.cfg
+++ b/openbsc/doc/examples/osmo-bsc_mgcp/mgcp.cfg
@@ -12,6 +12,7 @@
   bind ip 127.0.0.1
   bind port 2427
   rtp base 4000
+  rtp force-ptime 20
   sdp audio payload number 98
   sdp audio payload name AMR/8000
   number endpoints 31