gprs-ns/test: Re-add return value check for gprs_nsvc_reset

The return value check has been removed in the commit 'Use
gprs_nsvc_reset instead of gprs_ns_tx_reset', because gprs_nsvc_reset
had no return value then. Since a compatible return value has been
added by 'Let gprs_nsvc_reset return a value', the former patch is
partly reverted by this commit.

The printf is now an OSMO_ASSERT. The commit message was taken
from Jacob.

Sponsored-by: On-Waves ehf
gprs-ns/test: Check the nsvc state explicitely

This patch extends test_sgsn_reset_invalid_state by assertions to check
that nscv->state and nsvc->remote_state have been set as expected.

Sponsored-by: On-Waves ehf