RFR: 7902934: jtreg plugin should be friendly with different packaging layouts

[Maurizio Cimadamore]

This fix allows the plugin to work better across different versions of jtreg, as well as across different platforms.
Sometimes testng and jcommander are bundled in the same jar, sometimes in separate jars. This patch simply walks through all the files with a `.jar` extension in the jtreg folder and adds them to the classpath of the IDE test configuration.

-------------

Commit messages:
 - Add logic for discovering all jars in the jtreg lib folder

Changes: https://git.openjdk.java.net/jtreg/pull/10/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=10&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7902934
  Stats: 10 lines in 1 file changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/10.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/10/head:pull/10

PR: https://git.openjdk.java.net/jtreg/pull/10