Rename internal WhiteBoxTest annotation to ClearBoxTest

[sleberknight]

kiwi has its own annotation to indicate that a test is using internal methods, non-public APIs, etc. It was originally copied from kiwi-test. But after that happened, kiwi-test renamed it to @ClearBoxTest, so we should do that here as well for consistency. It has no impact except as an internal refactoring, since it is in the src/test directory and thus is not included in releases.