Added tests for _join methods
Added tests for _join methods

 - PathCombinator._join_up_segment
 - PathCombinator._join_core_segment
 - PathCombinator._join_down_segment
 - PathCombinator._join_up_segment_shortcuts
 - PathCombinator._join_down_segment_shortcuts
 - PathCombinator._join_shortcuts_peer

Oncilla/scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,netsec-ethz/scion,klausman/scion,dmpiergiacomo/scion,klausman/scion,netsec-ethz/scion,FR4NK-W/osourced-scion,klausman/scion,netsec-ethz/scion,Oncilla/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,klausman/scion,Oncilla/scion,dmpiergiacomo/scion,klausman/scion,netsec-ethz/scion,FR4NK-W/osourced-scion,Oncilla/scion