RFR JDK-8165564: langtools\test\jdk\jshell\CommandCompletionTest.java fails on some windows

[Jan Lahoda]

Hi,

It seems the completion and the tests sometimes don't agree on valid 
default FileSystem roots. This patch fixes that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8165564
Webrev: http://cr.openjdk.java.net/~jlahoda/8165564/webrev.00/

Any feedback is welcome!

Thanks,
     Jan