Don’t spy on the unit under test. Tests are meant to prove the existance of a given behaviour of a unit of logic (or disprove non-existance), changing the unit while testing it is counter productive.
private implementation details need to stay private. If you feel inclined to make something visible for testing, consider refactoring to simplify the interface you are testing. Consider adding more components in order to have smaller chunks to test.
Tests are the first consumer of the code. Writing the interface first can help simplify your testing, but can also help simplify usage of the unit in the future.
Tests exhibit the behaviour of a unit, and thus make great examples for those working in the same code base. Aim to exhibit all behaviours of a unit, don’t exclusively aim for code coverage.