Fix java.nio.file.Path.relativize on Windows (#3299)

* WindowsPath: Correct detection of the path type and the extraction of a root for DriveRelative and DirectoryRelative paths

* WindowsPath: Correct parent extraction, consider DirectoryRelative

* WindowsPath: Correct path creation in `relativize`

* WindowsPath: Empty path's type should be Relative

* [WIP] `relativize` should throw exceptions on invalid input

* WindowsPath: add unit tests

Tests are based on UnixPathTest with modifications and additions to
for Windows paths.
Tests were verified by running them vs JVM on Windows.
Some additional tests are commented out to mark known issues and help
with future development.

* WindowsPath: better switching of tests on JVM 8; add assertion IDs

Added IDs, so it is possible to identify which assertion is failing
just from the test output.