RFR: 8183505: Update langtools tests to allow for unique test classes directory

[Jonathan Gibbons]

jtreg has been updated via CODETOOLS-7902003 [1] to use a unique 
test.classes directory. This is part of improving the reliability when 
running tests concurrently. The change exposes issues in a small number 
of tests in the hotspot, jdk, and langtools repos and these issues will 
need to be fixed before the requiredVersion can be bumped to use a 
promoted jtreg with this change.

The few changes are all related to deriving the path for the library 
classes from test.class.path, instead of assuming a fixed relationship 
to the test's classes.

I'd like to push these to jdk10/jdk10 so that they can meet up with 
changes coming for related issues in the jdk and hotspot repos.

JBS: https://bugs.openjdk.java.net/browse/JDK-8183505
Webrev: http://cr.openjdk.java.net/~jjg/8183505/webrev.00/

-- Jon