We were testing modified_tags.args before we called get_twister_opt.py
so it would never have been created and thus twister_exclude_tag_opt
would never get set correctly. Move the check to after we call
get_twister_opt.py
Also we need to remove modified_tags.args once we are done with it.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>