mgcp/test: Reset pkt_period_* after each test

Currently these value may leak into the next test.

This patch will reset them after each message has been processed.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/mgcp/mgcp_test.ok b/openbsc/tests/mgcp/mgcp_test.ok
index d5fb56f..43a7402 100644
--- a/openbsc/tests/mgcp/mgcp_test.ok
+++ b/openbsc/tests/mgcp/mgcp_test.ok
@@ -45,7 +45,7 @@
 Connection mode: 2, BTS output disabled, NET output disabled
 Testing DLCX
 Detected packet duration: 20
-Requested packetetization period: 20-20
+Requested packetization period not set
 Connection mode: 0, BTS output enabled, NET output enabled
 Testing CRCX_ZYN
 Packet duration not set