Make rpmbuild run with local en_US.UTF-8 in tests
    
    This is necessary because different locales make rpmbuild output different
    text. For example, setting LANG=C, rpmbuild outputs
    
      Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.sRiekk
    
    However, setting LANG=cs_CZ, rpmbuild outputs following instead of
    Executing(%prep)
    
      Prov%prep): /bin/sh -e /var/tmp/rpm-tmp.E5Uk9p
    
    With this change, tests don't depend on the locale set in the host.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>