Field boundary finding logic issues

[joelposti]

Added unit tests that prove that the current field boundary finding logic causes a busy exhausted loop when user code calls continue and an error when user code calls break when an empty field data chunk is encountered.

More detailed explanation in issue Field boundary finding logic causes a busy exhausted loop.

[joelposti]

I first wrote these tests as integration tests. But I converted them to unit tests since it looks like the repo has zero integration tests. If you want these as integration tests instead, please let me know.