[fix] selenium: binary is not a Firefox executable

Since Ubuntu installs firefox frrom snap, we can't use any longer
``Browser('firefox')``.

[1] https://github.com/searxng/searxng/pull/3663#issuecomment-2255963036
[2] https://splinter.readthedocs.io/en/latest/drivers/firefox.html#custom-binary-path

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

tests/robot/__main__.py

@@ -7,6 +7,7 @@ import os
 import subprocess
 import traceback
 import pathlib
+import shutil
 
 from splinter import Browser
 
@@ -53,6 +54,9 @@ class SearxRobotLayer:
 
 def run_robot_tests(tests):
     print('Running {0} tests'.format(len(tests)))
+    print(f'{shutil.which("geckodriver")}')
+    print(f'{shutil.which("firefox")}')
+
     for test in tests:
         with Browser('firefox', headless=True, profile_preferences={'intl.accept_languages': 'en'}) as browser:
             test(browser)