fix failing jdk8 windows ci test

- AvailableJavaHomes fixture didn't understand symlinks on windows created with mklink
- comparing java-home using File#canonicalPath is not good enough for mklink symlinks
- use Files.isSameFile(Path, Path) if using jdk7 or newer now.
- at the moment all this logic lives in internal-testing. At one point we need to add better symlink
support (in general and on windows) in our native support.