tst-fs-link.so: Test case for link if source does not exist

Check that link returns ENOENT if source does not exist.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tst-fs-link.so: Add test case for link() if destination exists

Check that link returns EEXIST if destination exists.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>