gbproxy_test: Fix test expectations of gprs_ns_rcvmsg()

gprs_ns_rcvmsg() in old libosmocore returns "number of bytes transmitted
by any response PDU we sent as a result of the received message", while
modern libosmocore simply retunrs '0' for any successfully received
message.  Let's make sure any non-negative responses lead to a
reproducible test output with both old and new libosmocore.

Change-Id: I7a48d14aed19825b87a02ccf9ee9cbfe0853342c
2 files changed