Add option to check if a cached image isn't compared to an image during all tests

[MatthewFlamm]

It would be very useful to have an option to check that all cached images were compared during a test. This is a complement to checking that all comparisons have an image in the cache.

This is tricky if tests are skipped for any reason. So, it would be only useful if there is a canonical version for which no test is skipped. I'm not sure if this is the case today for pyvista. It is very possible that at least one plotting test is skipped on every vtk version. But, this could be useful for downstream libs that have less wide-open vtk version support.

[germa89]

I believe this is fail_extra_image_cache argument is for... but I also believe it is fundamentally broken...

If I set fail_extra_image_cache=True, it should fail all the tests with RegressionFileNotFound (See #80)

pytest-pyvista/pytest_pyvista/pytest_pyvista.py

Line 197 in 0ad5b8c

and self.fail_extra_image_cache

[germa89]

I think to have that feature, we are going to have to use os.listdir to cache the the image cache file names, and then at each test, pop each item.

The problem is to check at the end of the tests that there is no leftover.

[germa89]

This might be a solution:

https://stackoverflow.com/a/35394239/6650211

[MatthewFlamm]

I believe this is fail_extra_image_cache argument is for... but I also believe it is fundamentally broken...

I think it is a poor name for this argument. I think it does what the help message says

pytest-pyvista/pytest_pyvista/pytest_pyvista.py

Lines 23 to 27 in 0ad5b8c

	group.addoption(
	"--fail_extra_image_cache",
	action="store_true",
	help="Enables failure if image cache does not exist.",
	)

[germa89]

I would propose to change the name then.