Display relative image paths when tests fail.

[anntzer]

Before the PR, an image comparison failure was reported as

E   matplotlib.testing.exceptions.ImageComparisonFailure: images not close: /home/antony/src/extern/matplotlib/result_images/test_axes/pcolormesh_svg.png vs. /home/antony/src/extern/matplotlib/result_images/test_axes/pcolormesh-expected_svg.png (RMS 0.001)

This PR changes it to

E   matplotlib.testing.exceptions.ImageComparisonFailure: images not close (RMS 0.001):
E       result_images/test_axes/pcolormesh_svg.png
E       result_images/test_axes/pcolormesh-expected_svg.png

which I believe more legible.

(It would be nice to just display a single path but parts of the testing
machinery seem to accept arbitrary image paths, not necessarily in the
same folder, even though this is not done in practice.)

[dstansby]

I think it would be good to keep the full path instead of the relative path from the matplotlib directory, but splitting the two paths onto new lines is much better.

[madphysicist]

@dstansby. Technically, I think it is printing the path relative to the user's working directory, which will of course be the root matplotlib directory in 99% of cases. Perhaps just printing another line with the absolute path of the current directory would be sufficient?

[jkseppan]

When run in pytest (example) you get the full paths in the output in any case, so no information is lost by showing just the relative paths, and those will fit better on the screen so will probably be easier to select and copy. I think this is an improvement.

[NelleV]

Thanks @anntzer ! That's gonna be useful.