feat: simplify vm::testing::Contract

[loloicci]

Description

This PR simplifies vm::testing::Contract and makes it not take too much ownership of Backend.

This enables to specify the used querier and APIs in each generating instance from Contract. For example, using #180 's update_wasm after constructing a contract needs this PR.

Types of changes

• Bug fix (changes which fixes an issue)
• New feature (changes which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• ETC (build, ci, docs, perf, refactor, style, test)

Checklist

• I followed the contributing guidelines.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• The PR title and commits are followed conventional commit form.

[brew0722]

in general, LGTM.
However, during the review, I was confused as to why this PR is need.
I think it would be good to explain in the this PR body that the purpose is to easily reuse the same db in the PR below.

https://github.com/line/cosmwasm-simulator/pull/21/files#diff-42cb6807ad74b3e201c5a7ca98b911c5fa08380e942be6e4ac5807f8377f87fcR373

[loloicci]

I think it would be good to explain in the this PR body that the purpose is to easily reuse the same db in the PR below.

Thank you @brew0722. The point is not only reusing DB but making instances with specified querier and APIs. I updated the descryption.