test-socket-util: Add tests for receive_fd_iov() and friends.
    
    Test it when sending an FD without any contents, or an FD and some contents,
    or only contents and no FD (using a bare send().)
    
    Also fix the previous test which forked but was missing an _exit() at the
    end of the child execution code.
    
    (cherry picked from commit 8a3386ab4fea9c4efa9c72e7c149cf510a46f03e)
    
    Resolves: #1683319
    
    patch_name: 0189-test-socket-util-Add-tests-for-receive_fd_iov-and-fr.patch
    present_in_specfile: true
    location_in_specfile: 189
    squash_commits: true