RFR 8141609: Need test for jrtfs that runs on JDK 8 to target a JDK 9 image

[Felix Yang]

Please review the following new test.

Webrev:
http://cr.openjdk.java.net/~xiaofeya/8141609/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8141609

jdk 9 bundled a "jrt-fs.jar" file which expose the jrt filesystem 
support. It is critical that a tool on JDK 8 can use the jrt file system 
to target a JDK 9 image. This test is trying to address it.
Since jtreg doesn't support a clean way to work with multiple JDK 
releases. I leverage the JT_JAVA, which is usually configured with JDK 
8, as a workaround to achieve some automated tests without any 
customization on test environment.
Basic jrt file system testing in JrtfsTestMain.java are mainly covered 
with jdk/internal/jrtfs/PathOps.java and duplicated tests from 
jdk/internal/jrtfs/Basic.java.

Thanks,
Felix