tests: make test_goal_selector_upgrade() less assuming.

The test fails against libsolv after f190dfe because then libsolv keeps the
installed multiarch packages ('pilchard' in the test) in lockstep and can not
find the i686 alternative of the requested upgrade target.
fix Reldep inicialization without sack crash (RhBug:1052947)
fix not accepting numeric version in reldep (RhBug:1052961)
added nevra C API
added subject C API