baseapp test coverage not enough

[ebuchman]

• Test we can load old versions (ie. https://github.com/cosmos/cosmos-sdk/blob/develop/baseapp/baseapp_test.go#L56 doesn't actually do it there).
• Test that baseapp correctly handles gas on aborted txs #3259
• Let's get the coverage to 100% :)

[ValarDragon]

For getting test coverage to 100%, how should CheckTx be checked? In the tests that are currently there, and the ones I wrote in the PR, its using Check not the direct CheckTx method.

[ValarDragon]

The parts that remain untested:
run forever in helpers.go

TraceKVStore functionality
RegisterCodespace
MountStoreWithDB
LoadVersion errors
Baseapp.NewContext
Filter errors
Bad query paths
Version queries

[alexanderbez]

Thanks for aggregating this @ValarDragon