tests: don't fail with newer gnutls
    
    gnutls-3.3.0 and newer leaves 2 FDs open in order to be backwards
    compatible when it comes to chrooted binaries [1].  Linking
    commandhelper with gnutls then leaves these two FDs open and
    commandtest fails thanks to that.  This patch does not link
    commandhelper with libvirt.la, but rather only the utilities making
    the test pass.
    
    Based on suggestion from Daniel [2].
    
    [1] http://lists.gnutls.org/pipermail/gnutls-help/2014-April/003429.html
    [2] https://www.redhat.com/archives/libvir-list/2014-April/msg01119.html
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>