There are some internal JRT file-system classes very likely to undergo non trivial changes in preparation for Valhalla, but which currently don't have specific unit tests (they are well tested by the JDK overall, but it will be hard to reason about any issues when making changes).

Specifically, files in `jdk.internal.jrtfs`, including:
* SystemImage
* ExplodedImage

I'd like to add targeted (white box) unit tests for these in the JDK mainline so that the tests are exercised and shown to pass now, and any common refactoring which might go into mainline as part of my work gets test coverage before and after any changes.