Drop rpmfluff in test
    
    rpmfluff builds packages inside base_dir returned from get_base_dir.
    This causes TestImportSrpm fails when run with detox or other ways in
    parallel. This patch drops rpmfluff and builds SRPM for that test
    directly with rpmbuild.
    
    Relates: #288
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>