testpbx: Enable GSM and AMR as codecs to actually be relevant

Without these codecs we will not have much joy when working with a
default GSM BTS/BSC.

Change-Id: I8719c392f57e0dbed0c0c792d1381592dcd662c1

contrib/testpbx/configs/vars.xml

@@ -255,8 +255,8 @@
        127 - BV32
 
   -->
-  <X-PRE-PROCESS cmd="set" data="global_codec_prefs=OPUS,G722,PCMU,PCMA,VP8"/>
-  <X-PRE-PROCESS cmd="set" data="outbound_codec_prefs=OPUS,G722,PCMU,PCMA,VP8"/>
+  <X-PRE-PROCESS cmd="set" data="global_codec_prefs=OPUS,G722,PCMU,PCMA,VP8,GSM,AMR"/>
+  <X-PRE-PROCESS cmd="set" data="outbound_codec_prefs=OPUS,G722,PCMU,PCMA,VP8,GSM,AMR"/>
 
   <!--
       xmpp_client_profile and xmpp_server_profile