Remove deprecated call to platform.dist
    
    As documented in Python documentation, platform.dist is deprecated
    since version 2.6 and platform.linux_distribution should be used in new
    code. Meanwhile, Python 2.6 is already the minimum version fedpkg works
    with. It makes sense to remove the call to platform.dist at current
    stage.
    
    Tests are added and new code for writing and running tests are also
    added.
    
    This patch also fixes a bug that is original mydist is referenced
    before assignment.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>