[ENH TEST] Improve compare_folders error message

[NicolasGensollen]

This PR proposes to improve the error message of the testing utility function compare_folders.

The function currently returns True if the two folders have the same file trees and raises a ValueError otherwise. The error message contains both trees.

It is easy to spot differences when the folders are not too large/deep, but quickly becomes difficult when they increase in size.

For instance, I'm having troubles seing the differences when non-regression tests for converters fail as the folders are quite large for these pipelines.

This PR adds a section at the end of the error message: it gives the number of lines which differ between the two trees and it prints the differences explicitly.

Here is a small example:

>>> compare_folders(out_folder, ref_folder, tmp_path)
E           ValueError: Comparison of out and ref directories shows mismatch :
E            The content of the OUT directory :
E               + subfolder1
E                   + file1.txt
E               + subfolder2
E                   + file2.xt
E               + subfolder3
E                   + file3.xt
E
E            The content of the REF directory :
E               + subfolder1
E                   + file1.txt
E               + subfolder3
E                   + file4.xt
E               + subfolder4
E                   + file2.xt
E
E            There are 4 lines with a mismatch :
E           		- subfolder2 != subfolder3
E           		- file2.xt != file4.xt
E           		- subfolder3 != subfolder4
E           		- file3.xt != file2.xt