Use memdb in ORM tests

[blushi]

ref: #10481 (comment)

Use github.com/cosmos/cosmos-sdk/db/memdb in x/group/internal/orm tests instead of the current setup:

ctx := NewMockContext()
store := ctx.KVStore(sdk.NewKVStoreKey("test"))

[tac0turtle]

I couldn't find this line of code in orm. Will this code land in a later pr?

[tac0turtle]

closing this as the orm has changed quite a bit since this issue was opened

[blushi]

This was for the old version of the ORM that lives in x/group/internal/orm (currently still being used in x/group)
But not sure how important that is now since we'll eventually use the new ORM implementation.