support/testing: add fs tests

This commit adds a number of test cases for various filesystem formats:
ext2/3/4, iso9660, jffs2, squashfs, ubi/ubifs and yaffs2. All of them
except yaffs2 are runtime tested. The iso9660 set of test cases is
particularly rich, testing the proper operation of the iso9660 support
with all of grub, grub2 and isolinux.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>