Duplicate-aware testing mechanism
Duplicate-aware testing mechanism

The old extra_entry_points couldn't register two classes under the same
identifier. We're going to need that for testing.

lovehhf/xblock-sdk,nagyistoce/edx-XBlock,stvstnfrd/xblock-sdk,lovehhf/xblock-sdk,edx-solutions/xblock-sdk,EDUlib/XBlock,mitodl/XBlock,Pilou81715/hackathon_edX,nagyistoce/edx-xblock-sdk,nagyistoce/edx-xblock-sdk,Lyla-Fischer/xblock-sdk,nagyistoce/edx-XBlock,4eek/XBlock,jamiefolsom/xblock-sdk,dcadams/xblock-sdk,open-craft/XBlock,jamiefolsom/xblock-sdk,edx/xblock-sdk,lovehhf/XBlock,cpennington/XBlock,4eek/XBlock,EDUlib/XBlock,edx-solutions/xblock-sdk,Lyla-Fischer/xblock-sdk,nagyistoce/edx-xblock-sdk,edx/XBlock,edx-solutions/xblock-sdk,cpennington/XBlock,dcadams/xblock-sdk,Pilou81715/hackathon_edX,edx-solutions/XBlock,lovehhf/xblock-sdk,lovehhf/XBlock,edx/xblock-sdk,dcadams/xblock-sdk,open-craft/XBlock,jamiefolsom/xblock-sdk,stvstnfrd/xblock-sdk,lovehhf/xblock-sdk,edx-solutions/XBlock,edx/xblock-sdk,mitodl/XBlock,jamiefolsom/xblock-sdk,nagyistoce/edx-xblock-sdk,Lyla-Fischer/xblock-sdk,edx/XBlock,stvstnfrd/xblock-sdk,Pilou81715/hackathon_edX,edx-solutions/xblock-sdk,cpennington/XBlock,Pilou81715/hackathon_edX