salesforce: deep dive on unit tests

[josephjclark]

Once the new salesforce API is stabilised, I want to do a deep dive on unit testing

We should use unidici mocks to mock out the requests.

I want a really clean mock API with good clean, re-usable mock data. I am tempted to bring #482 into this, although there's unrelated stuff in that PR.

I want to set a standard for unit tests on core adaptors here. We should test the happy path and different options, expand references, and error handling.

Every Operation should have a bunch of tests:

• basic functionality (ie, writes to state.data)
• expandable references
• any particularly complex options need a test
• common error patterns

I probably wouldn't bother testing references

Tests should be small, focused and precise. Each test should test exactly one thing, and do it thoroughly