Fix tests for RHEL5+, except for RHEL8
    
    - put dnf-plugins-core and yum-utils into Requires in Makefile (it
      is OK to put non-installable package names there?)
    - add builddep() handler which accepts package names;  this isn't
      working with spec file or SRPM file, but only with package names
      but it doesn't matter much (we take srpm from repository
      anyways, so using source repository for build deps is fine as
      well).
    - on rhel5/rhel6 is /bin/tar, not /usr/bin/tar;  so use /bin/tar
      everywhere to keep things simple
    - better prepare $HOME for 'tar-testsuite' user on RHEL5