tests: don't fail with newer gnutls

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>