tests: When checking squashfs fstime, use superblock last modified time

tests: When checking squashfs fstime, use superblock last modified time

Currently, the filesystem timestamp check in grub-fs-tester uses the
squashfs image file's last modified timestamp and checks to see if that
time stamp is within 3 seconds of the superblock timestamp as determined by
grub. The image file's timestamp could be more than 3 seconds off if
mksquashfs takes more than 3 seconds to generate the image, as is the case
on a virtual machine. Instead use squashfs tools to get the filesystem
timestamp directly.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>