Plot unit tests do not include MacOS and Ubuntu files

[jamestripp]

Unit tests for plots check postscript output for of a plot against postscript files where the plot was visually checked and accurate. However, postscript files appear to differ depending on platform. At present the tests return true when run on MacOS and Ubuntu.

Postscript files under MacOS and Ubuntu need to be added and these files loaded and checked when the test is run.