MastForest: Add convenience method for tests

[plafer]

Originally posted by @bobbinth in #1349 (comment)

Not for this PR, but I see this pattern a lot in tests. I wonder if it makes sense to create convenience methods on MastForest which would create a node and add it in a single call. For example, something like:

let last_basic_block_id = mast_forest.ensure_basic_block(vec![Operation::FmpAdd]);

[sergerad]

Happy to do this one 👍

[bobbinth]

Closed by #1404.