Create a db_home directory with a unique name so multiple users can

Create a db_home directory with a unique name so multiple users can
run the test simultaneously.  The simplest thing I found that worked
on both Windows and Unix was to use the PID.  It's unique so should be
sufficient.  This should prevent many of the spurious failures of
the automated tests since they run as different users.

Also cleanup the directory consistenly in the tearDown methods.

It would be nice if someone ensured that the directories are always
created with a consistent name.