parted: skip tests that need vfat support

At the time of writing the qemu kernels don't support vfat filesystems.
There are patches on the list to add that, but as two tests fail without
vfat support, make them skip if vfat isn't available.

[ YOCTO #14470 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>