tests: exclude some tests when running on NFS

tests: exclude some tests when running on NFS

All tests currently pass on NFS on Linux kernel 2.6.22 at least,
but some fail on 2.6.9, so we exclude those here.

* tests/init.cfg (is_local_dir_): A new function
returning if the specified directory is on a local file system.
(require_local_dir_): A new function to skip tests
if the current directory is not on a local file system.
* tests/cp/existing-perm-race: Skip if non local.
* tests/cp/file-perm-race: Likewise.
* tests/cp/parent-perm: Likewise.
* tests/cp/parent-perm-race: Likewise.
* tests/cp/preserve-2: Likewise.
* tests/mv/part-symlink: Likewise.
* tests/du/basic: Use refactored function.
* tests/install/basic-1: Likewise.
* tests/mkdir/p-3: Likewise.
* tests/dd/skip-seek-past-dev: Likewise.
* tests/du/slink: Likewise.  Remove redundant test
for NFS file system.
* tests/misc/join: s/local/locale/.