gprs/test: Add BSSGP tests

This patch adds a test suite for the BSSGP protocol.
The first (and only) test checks the handling of BSSGP
SUSPEND/RESUME.

Sponsored-by: On-Waves ehf
gprs: Don't discard SUSPEND/RESUME in bssgp_rcvmsg

Currently sending SUSPEND/RESUME messages to this function (like it
is done in the osmo-sgsn) results in STATUS messages complaining
about an unknown BVCI. The reason is, that these messages rely on a
TLLI/RAI pair to identify the context and do not contain an explicit
BVCI.

This patch modifies bssgp_rcvmsg() to only complain about and unknown
BVCI if one is given but a matching context is not found (except for
RESET messages). The ctx argument is removed from the functions
handling SUSPEND and RESUME since it will always be NULL then.

Sponsored-by: On-Waves ehf