Add sub-store unit tests (#246)
Add sub-store unit tests (#246)

This commit adds unit tests against mocked stores for most
of the public API of the sub store package. Those are unit
and not integration tests, but still:

Fixes #90
gopasspw/gopass,gopasspw/gopass,metalmatze/gopass,justwatchcom/gopass,franciscocpg/gopass,metalmatze/gopass