fs-test: Add test for a filename using '..' to go back to the root

The previous commit fixed a problem in FAT code where going back to the
root directory using '..' wouldn't work correctly on FAT12 or FAT16.
Add a test to exercise this case (which was once fixed in commit
18a10d46 "fat: handle paths that include ../" but reintroduced due to
the directory iterator refactoring).

This test only very barely catches the problem - without the fix the
size command still gives valid output but the additional spurious
"Invalid FAT entry" error message makes it not get caught in the
'egrep -A3 ' output. I tried to make a proper test that grows the root
directory to two clusters lots of with dummy files but that causes the
write tests to crash the sandbox totally...

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Tom Rini <trini@konsulko.com>