Add tests for API package

[Maelkum]

@dmikey Inspired by your comment that there's a bug in how the API behaves on function install code path, I figured it was good to add tests to the API package as well, so we don't have any surprises.

This PR adds tests to the API package, with the test coverage for the package now at 97.7%.

Tests typically cover three scenarios:

1. nominal case - everything works as expected
2. error - one of the node methods tasked with doing something failed
3. invalid payload - e.g. malformed JSON