test-dup2: reduce dependencies

test-dup2: reduce dependencies

dup2 is fully tested if the cloexec module is in use, but it is
also used in a number of modules where dragging in cloexec is
over the top for just using dup2 to test whether an fd is valid.

* modules/cloexec (Configure.ac): Set witness.
* modules/dup2-tests (Depends-on): Drop cloexec.
* tests/test-dup2.c (main): Skip portion of test if cloexec module
not present.
Suggested by Bruno Haible.

Signed-off-by: Eric Blake <ebb9@byu.net>