Create a test to check that all images in the cache are utilized by a test

[MatthewFlamm]

Describe what maintenance you would like added.

#3336 added a image cache file for closing_and_mem_cleanup.png despite the fact that this test does not cache a plotter image. In fact, the image should have a basic sphere in this test, so the image is also wrong.

pyvista/tests/plotting/test_plotting.py

Lines 1844 to 1852 in bf82d18

	def test_closing_and_mem_cleanup():
	n = 5
	for _ in range(n):
	for _ in range(n):
	p = pyvista.Plotter()
	for k in range(n):
	p.add_mesh(pyvista.Sphere(radius=k))
	p.show()
	pyvista.close_all()

It would be useful to have a test, in some form, that ensures that all images are referenced in a test to avoid this kind of problem in the future.

[MatthewFlamm]

A complicating factor to this is that some tests only cache images on certain vtk versions.