Document directory name config settings

[clach04]

Temporary directories used by tests are currently hard coded (see readme), this is less than ideal.

Add support to override this globally (e.g. pick up from II_TEMPORARY?) in the framework so tests can avoid duplicating code.

[clach04]

OR_UNITTEST_TMPDIR controls temp directories and needs to be updated. Also see other options, e.g. OR_UNITTEST_RESOURCEDIR