Test spec with Cucumber

[aakoshh]

We could compile IPC test scenarios in Gherkin syntax (that is, plain english) and run it with Cucumber, as I demoed in filecoin-project/ref-fvm#1507

This would allow us to capture what should happen independent from specification documents and Go/Rust tests. Since we want to implement IPC with different tech stack (e.g. Eudico and Fendermint), it could be the basis of a test suite that all implementations should be able to pass. The developer just has to implement the "matchers".

We can use it to describe scenarios such as:

When account 1 in subnet 1 sends 10 tokens to account 2 in subnet 2
And the checkpoint from subnet 1 is processed
Then account 2 in subnet 2 has 10 tokens

When account 1 in rootnet withdraws their delegation from validator 1 in subnet 1
Then subnet 1 goes under the minimum collateral threshold in rootnet
And subnet 1 is suspended in rootnet

See:

• Solidity contract features and their implementation
• https://cucumber-rs.github.io/cucumber/current/quickstart.html