Improve testing infrastructure for Python code.

The build system now correctly finds a list of source folders (for each component/project)
containing .py files, and will set PYTHONPATH for tests to include these, so tests no
longer need to do any munging of sys.path themselves.

As a side effect, all tests will now be run in ''just'' the environment set up by SCons,
which should have been the case anyway, but I'm now more confident it's correct!

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19010 b5f69653-2bf5-0310-8201-b0dafb8e1405