Add -nativepath flag

I would like to see the addition of a flag to specify the path of the test binaries. I’m thinking something like “-natives:<path>”. This would do two things:

- Set the java.library.path when invoking tests. This is needed for System.loadLibrary() to work.
- Set a test.natives property. This is needed in the cases where the test binary is not a library, but a program. In this case the test will want to launch that program and needs the path to the program.

Both of these would be set to <path> above. Jtreg should check that the path exists.

See this email thread for a discussion: http://mail.openjdk.java.net/pipermail/jtreg-dev/2014-April/000107.html