tests: reduce dependencies

tests: reduce dependencies

We can assume that tests won't be run from a directory with an
absolute name longer than native getcwd limits.

* tests/test-linkat.c (main): Use lighter-weight getcwd.
* tests/test-renameat.c (main): Likewise.
* modules/linkat-tests (Depends-on): Relax dependency.
* modules/renameat-tests (Depends-on): Likewise.
* modules/fchdir-tests (Depends-on): Likewise.  Also make cloexec
dependency explicit.

Signed-off-by: Eric Blake <eblake@redhat.com>