Changes

mgcp_test: test a=ptime:20, not 40

In our GSM world, the packet time is 20 ms, hence 20 is the typical
value that we should test. Maybe we can do one test with 40 at most, but
we'll most probably never see anything else than 20 in practice.

Change-Id: Ia2292198bf6e3b72912afd69607654ca77fd549d