Add example for mocking chain extensions in off-chain tests

[cmichi]

No description provided.

[frankli-dev]

Do I need to mock chain extension inside every test function? Is there anyway to define outside and use that in all functions?

[cmichi]

You can also define it outside, I just mocked it in here since it's only used in this one test.

[frankli-dev]

I'm getting the following error

862 |             impl ink_env::test::ChainExtension for MockedExtension {
    |                                 ^^^^^^^^^^^^^^ private trait
    |
note: the trait `ChainExtension` is defined here
   --> /Users/mac/.cargo/registry/src/github.com-1ecc6299db9ec823/ink_env-3.0.0-rc3/src/engine/off_chain/test_api.rs:18:5
    |
18  |     chain_extension::ChainExtension,

I think, this PR covers the above issue but does it mean that once the PR is merged, I need to use the latest ink?

[cmichi]

I think, this PR covers the above issue but does it mean that once the PR is merged, I need to use the latest ink?

@frankli-dev Yes, until we release the next release candidate you need to reference the git repository for your Cargo.toml dependencies. The error you get is because this PR was not merged yet, after it's merged it will work.