Update unstructured grid page examples, add them to test suite

[sadielbartholomew]

As per title. These code snippets were missed out from the test suite of examples, probably because they were not numbered examples and I only added those on a first pass, so I've added a new test class for unnumbered examples (those named but not assigned a number such as 'Example N' for some N) which I will populate soon.

Follow-on work:

• check for unnumbered examples across the docs and add all of these to the test suite, then ensure they work and coordinate so the precise working versions are documented;
• add reference plots to compare with, the uncomment the @compare_plot_results decorators
• investigate two test failures (I fixed up the setup but the final plot still fails though it shouldn't in these cases), one of which seems to be a cf-plot bug.

[sadielbartholomew]

Merging through so I can begin follow-on work and other work towards a new release.