#2016 - fix a `continuum_mechanics` test

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20271 b5f69653-2bf5-0310-8201-b0dafb8e1405
Fix coverage in r20263/#2146

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@20272 b5f69653-2bf5-0310-8201-b0dafb8e1405