Add test coverage for KeyMemoryStore (and by extension, MemoryFileStore)
Add test coverage for KeyMemoryStore (and by extension, MemoryFileStore)

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

aduermael/docker.github.io,dnascimento/notary,JimGalasyn/docker.github.io,docker-zh/docker.github.io,johnstep/docker.github.io,joeuo/docker.github.io,mtrmac/notary,minhoryang/notary,troy0820/docker.github.io,denverdino/docker.github.io,gdevillele/docker.github.io,denverdino/docker.github.io,joaofnfernandes/docker.github.io,jfrazelle/notary,HuKeping/notary,docker/notary,rillig/docker.github.io,londoncalling/docker.github.io,johnstep/docker.github.io,moxiegirl/notary,jzwlqx/denverdino.github.io,joeuo/docker.github.io,andrewhsu/notary,andrewhsu/notary,LuisBosquez/docker.github.io,bdwill/docker.github.io,anweiss/docker.github.io,mtrmac/notary,vikstrous/notary,vdemeester/notary,sanscontext/docker.github.io,dqminh/notary,shin-/docker.github.io,johnsandiford/notary,docker/docker.github.io,sanscontext/docker.github.io,docker/docker.github.io,riyazdf/notary,alexisbellido/docker.github.io,bdwill/docker.github.io,thaJeztah/docker.github.io,menglingwei/denverdino.github.io,JimGalasyn/docker.github.io,shubheksha/docker.github.io,shubheksha/docker.github.io,phiroict/docker,vdemeester/notary,johnsandiford/notary,lindenlab/notary,danix800/docker.github.io,londoncalling/docker.github.io,troy0820/docker.github.io,joeuo/docker.github.io,shin-/docker.github.io,shin-/docker.github.io,umayr/notary,troy0820/docker.github.io,docker/notary,denverdino/denverdino.github.io,rillig/docker.github.io,andrewhsu/notary,jzwlqx/denverdino.github.io,johnstep/docker.github.io,cyli/notary,LuisBosquez/docker.github.io,NathanMcCauley/notary,LuisBosquez/docker.github.io,anweiss/docker.github.io,londoncalling/docker.github.io,danix800/docker.github.io,lindenlab/notary,joeuo/docker.github.io,denverdino/denverdino.github.io,BSWANG/denverdino.github.io,aduermael/docker.github.io,shubheksha/docker.github.io,bdwill/docker.github.io,phiroict/docker,bgrissin/notary,sanscontext/docker.github.io,londoncalling/docker.github.io,docker-zh/docker.github.io,JimGalasyn/docker.github.io,cyli/notary,anweiss/docker.github.io,joaofnfernandes/docker.github.io,NathanMcCauley/notary,gdevillele/docker.github.io,thaJeztah/docker.github.io,mtrmac/notary,alexisbellido/docker.github.io,sanscontext/docker.github.io,gdevillele/docker.github.io,jbeda/notary,londoncalling/docker.github.io,joaofnfernandes/docker.github.io,moxiegirl/notary,joaofnfernandes/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,LuisBosquez/docker.github.io,riyazdf/notary,endophage/notary,denverdino/denverdino.github.io,denverdino/docker.github.io,aduermael/docker.github.io,JimGalasyn/docker.github.io,bdwill/docker.github.io,thaJeztah/docker.github.io,docker-zh/docker.github.io,docker/docker.github.io,docker/docker.github.io,HuKeping/notary,minhoryang/notary,rillig/docker.github.io,dqminh/notary,BSWANG/denverdino.github.io,SachinSetiya/notary,grunya404/notary,bgrissin/notary,sjfloat/notary,menglingwei/denverdino.github.io,shubheksha/docker.github.io,BSWANG/denverdino.github.io,denverdino/denverdino.github.io,rillig/docker.github.io,joeuo/docker.github.io,jbeda/notary,menglingwei/denverdino.github.io,troy0820/docker.github.io,jfrazelle/notary,joaofnfernandes/docker.github.io,johnsandiford/notary,cw2018/notary,docker/notary,HuKeping/notary,cyli/notary,docker/docker.github.io,BSWANG/denverdino.github.io,menglingwei/denverdino.github.io,NathanMcCauley/notary,alexisbellido/docker.github.io,JimGalasyn/docker.github.io,umayr/notary,phiroict/docker,johnstep/docker.github.io,jfrazelle/notary,vikstrous/notary,dnascimento/notary,phiroict/docker,danix800/docker.github.io,jzwlqx/denverdino.github.io,bdwill/docker.github.io,endophage/notary,sjfloat/notary,shin-/docker.github.io,docker-zh/docker.github.io,menglingwei/denverdino.github.io,LuisBosquez/docker.github.io,alexisbellido/docker.github.io,bsmr-docker/notary,shubheksha/docker.github.io,bsmr-docker/notary,endophage/notary,grunya404/notary,danix800/docker.github.io,sanscontext/docker.github.io,johnstep/docker.github.io,alexisbellido/docker.github.io,BSWANG/denverdino.github.io,umayr/notary,docker-zh/docker.github.io,shin-/docker.github.io,jzwlqx/denverdino.github.io,thaJeztah/docker.github.io,cw2018/notary,denverdino/denverdino.github.io,aduermael/docker.github.io,denverdino/docker.github.io,jzwlqx/denverdino.github.io,vdemeester/notary,phiroict/docker,SachinSetiya/notary,moxiegirl/notary,gdevillele/docker.github.io,gdevillele/docker.github.io,denverdino/docker.github.io,dqminh/notary,anweiss/docker.github.io