Add more tests for euler_cycle corner cases.
Add more tests for euler_cycle corner cases.

Sixshaman/networkx,farhaanbukhsh/networkx,beni55/networkx,nathania/networkx,OrkoHunter/networkx,ionanrozenfeld/networkx,dmoliveira/networkx,ghdk/networkx,ghdk/networkx,wasade/networkx,jfinkels/networkx,aureooms/networkx,jcurbelo/networkx,SanketDG/networkx,debsankha/networkx,harlowja/networkx,blublud/networkx,kai5263499/networkx,dmoliveira/networkx,blublud/networkx,kai5263499/networkx,sharifulgeo/networkx,blublud/networkx,dhimmel/networkx,RMKD/networkx,jni/networkx,aureooms/networkx,goulu/networkx,harlowja/networkx,jni/networkx,cmtm/networkx,jakevdp/networkx,ltiao/networkx,chrisnatali/networkx,yashu-seth/networkx,kernc/networkx,chrisnatali/networkx,jakevdp/networkx,kai5263499/networkx,ionanrozenfeld/networkx,farhaanbukhsh/networkx,michaelpacer/networkx,ghdk/networkx,jtorrents/networkx,ionanrozenfeld/networkx,jni/networkx,aureooms/networkx,farhaanbukhsh/networkx,kernc/networkx,nathania/networkx,nathania/networkx,sharifulgeo/networkx,NvanAdrichem/networkx,jtorrents/networkx,dmoliveira/networkx,dhimmel/networkx,harlowja/networkx,debsankha/networkx,kernc/networkx,tmilicic/networkx,andnovar/networkx,bzero/networkx,bzero/networkx,RMKD/networkx,bzero/networkx,RMKD/networkx,dhimmel/networkx,chrisnatali/networkx,JamesClough/networkx,debsankha/networkx,sharifulgeo/networkx,jakevdp/networkx