Fix running the tests with the perfplugin but without messing coverage
    
    With the way we were importing the perfplugin before running the tests
    we were messing with the coverage info.
    By just doing this tweak, we are increasing our coverage from 63% to 86%.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>