Improve spec file and show details about failing tests in output.
    
    - Fix License tag.
    - Package COPYING.txt and use it with license macro.
      (see libdfp-license.patch and upstream pull-request
      https://github.com/libdfp/libdfp/pull/86)
    - Remove Group and BuildRoot tags.
    - Use buildroot macro instead of RPM_BUILD_ROOT variable.
    - Do not remove buildroot during install step.
    - Use pushd/popd instead of cd commands.
    - Remove clean section.
    - Fix changelog in order to not include macros.
    - Print summary of failing tests in the output of make check.
      (see libdfp-tests.patch and upstream pull-request
      https://github.com/libdfp/libdfp/pull/87)
    
    Signed-off-by: Stefan Liebler <stli@linux.ibm.com>