Define testing strategy

[plafer]

We need to define a testing strategy that will identify what needs to be tested along with how to test it. For example, some questions to answer include

• Currently MockContext doesn't do proper proofs. Should it? This would involve modifying the store to be a Merkle tree.
• We want to make sure that we are compatible with gaia. However, integration tests of that sort are very slow, so we probably don't want to run them on every PR. Perhaps before every release is sufficient?
• What will we be testing with basecoin-rs, that won't be covered with the MockContext-based tests?

Perhaps the outcome of this issue is an ADR which answers all these questions.

[adizere]

There's an ADR linked from #1109 that closes this issue.

[adizere]

We have not answered the third question in the original issue description:

What will we be testing with basecoin-rs, that won't be covered with the MockContext-based tests?

This is subject for future work.