Harden partest against duplicate paths.

A serious issue: partest would launch the same test multiple
times if a test path was given in multiple forms (e.g. absolute
and relative paths.) Unfortunately all those tests would share
the same logfile, output directory, etc. which would predictably
lead to explosions.

Since overwriting classfiles while being loaded can lead to
jvm core dumps, it's possible this is involved in recent jvm
crashes and other test breakdowns.

This commit also alters the default partest verbosity to only
full print test transcripts under --verbose.
Merge pull request #2571 from paulp/pr/canonicalize-paths

Harden partest against duplicate paths.