Replace ReflectionTestUtils usage by hasFieldOrPropertyWithValue

[snicoll]

AssertJ has a nice way to assert private fields. We could consider moving to that rather than using ReflectionFieldUtils manually.

[philwebb]

Closing in favor of PR #14783

[mbhave]

I don't know if should close this. There are some more usages of ReflectionTestUtils.getField.

[snicoll]

Indeed