Update test and plugin description and add test

[anne-glerum]

To finalize addressing issue #4056, I've cloned the test that checks whether surface and volume stress visualization output give the same results to include set Point-wise stress and strain = true. With this setting the stresses should actually be equal.

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[bangerth]

I assume you have verified that the two really do agree?

[anne-glerum]

I assume you have verified that the two really do agree?

In the issue, I wrote that they are the same: "I've checked again now that #4181 is merged, and with point-wise values, the output on the surface mesh and the volume mesh is the same 🥳 ."

Do you want me to check again?

[bangerth]

No, I was just wondering because here you wrote "With this setting the stresses should actually be equal." I take your word that they are :-)

[anne-glerum]

While fixing the test, I also compared the output on the volume and on the surface mesh, and the stresses are indeed still the same.

[bangerth]

You win the prize for the most thoroughly thought through PR then! :-)