gprs-ns/test: Use gprs_nsvc_reset instead of gprs_ns_tx_reset

Currently gprs_ns_tx_reset is used to let the NS stack generate
NS_RESET message. This is not adjusting the nsvc state properly.

This patch uses gprs_nsvc_reset instead which starts the full reset
procedure.

Sponsored-by: On-Waves ehf
gprs-ns/test: Save the last PDU type sent

This records the PDU type of the last message sent to the remote peer
in the global variable sent_pdu_type, which can then be used in
assertions.

Note that sent_pdu_type will remain unchanged if no message has been
sent via sendto.

Sponsored-by: On-Waves ehf
gprs-ns/test: Add expire_nsvc_timer

This function forces the expiration of the timer within struct
gprs_nsvc. It is meant to be used for controlled (and faster) timer
expiration in unit tests. The timer_mode of the expired timer is
returned (or -1 if the timer wasn't active).

Sponsored-by: On-Waves ehf
gprs-ns/test: Add test_sgsn_reset_invalid_state

This tests the behaviour for a certain combination of messages and
packet losses that can put the NS-VC object into an undefined state.

In that state, the peer's ALIVE messages are properly answered, but
the local test procedure has stopped, incoming unitdata PDUs are
answered by STATUS(NS-VC blocked) and the unblocking procedure is not
initiated.

Ticket: OW#1312
Sponsored-by: On-Waves ehf
gprs-ns: Let gprs_nsvc_reset return a value

Currently gprs_nsvc_reset does not return any value.

This patch changes the function to return an integer, where a value
less than zero indicates an error. The value is taken from the
gprs_ns_tx_reset function. In case of failure, an error message is logged.

Sponsored-by: On-Waves ehf
gprs-ns: Fix reset state handling

Currently the NS-VC's state is updated from within gprs_ns_tx_reset,
which can lead to an inconsistent state when the RESET_ACK is lost.
In this state, the NSE_S_RESET bit is set but the Tns-reset timer is
not started.

This patch moves the state update into gprs_nsvc_reset. This way, the
state flags are consistent with the timer.

Addresses:
  SGSN -> BSS       NS_ALIVE
  BSS -> SGSN       NS_ALIVE_ACK
  BSS -> SGSN       BVC_RESET
  SGSN -> BSS       NS_STATUS, Cause: NS-VC blocked, NS VCI: 0x65
  and there is no BSS->SGSN NS_ALIVE

Ticket: OW#1213
Sponsored-by: On-Waves ehf