RFR: 8133245 Use external.lib.roots instead of relative paths for @library

[Staffan Larsen]

jtreg @library entries like these: 

 @library /../../test/lib 

which refer to classes in the top-level repo, can be changed to 

  * @library /test/lib 

if external.lib.roots=../../ is added to TEST.ROOT. This is a new feature in jtreg 4.1b12 (which we are currently using).

Please review the changes to the serviceability tests to incorporate this:

bug: https://bugs.openjdk.java.net/browse/JDK-8133245
webrev hotspot: http://cr.openjdk.java.net/~sla/8133245/hotspot/webrev.00/
webrev jdk: http://cr.openjdk.java.net/~sla/8133245/jdk/webrev.00/

Thanks,
/Staffan