finish gbproxy_parse_bssgp_unitdata test

The message this test is trying to parse is indeed invalid.
Add a comment showing the message in decoded form, and assert
that the parser rejects it.

Also, add a missing call to cleanup_test().

Change-Id: I2a86432d080c38d3c95626372a0129499d7146dd
Related: OS#3178
1 file changed