test: fs: Added tests for symlinks

Test cases are:
1) basic link creation, verify it can be followed
2) chained links, verify it can be followed
3) replace exiting file a with a link, and a link with a link. verify it
   can be followed
4) create a broken link, verify it can't be followed

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>