Fix coverage for r20970, and note why we need the `SetTimestep` method.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20990 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2441 - fix archive test on Windows

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20991 b5f69653-2bf5-0310-8201-b0dafb8e1405