What really helps in the case of testing is lead by example. If the project itself provides a good test framework and examples of properly implemented tests it

What really helps in the case of testing is lead by example. If the project itself provides a good test framework and examples of properly implemented tests it is easy for contributors to add tests as well. But asking people to figure this stuff out themselves is often a really big hurdle. Also parametrized tests. I find no shame in asking people to add a case to a existing parameterized testsuite when submitting an issue.