searx/testing.py: bugfix process model (do not use Flask's debug mode)

Don't enable Flask's debug mode, the debugger from Flask will cause a wired
process model, where the server never dies.  Further read:

- debug mode: https://flask.palletsprojects.com/quickstart/#debug-mode
- Flask.run(..): https://flask.palletsprojects.com/api/#flask.Flask.run

closes: https://github.com/asciimoo/searx/issues/1862



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>