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>