[fix] unit tests: call searx.search.initialize in test's setUp

Depending on the order the unit tests are executed, the searx.search module is
initalized or not, issue reported in [1]::

    Traceback (most recent call last):
      File "searxng/tests/unit/test_results.py", line 72, in test_result_merge_by_title
        self.container.extend('stract', [fake_result(engine='stract', title='short title')])
      File "searxng/searx/results.py", line 243, in extend
        histogram_observe(standard_result_count, 'engine', engine_name, 'result', 'count')
      File "searxng/searx/metrics/__init__.py", line 49, in histogram_observe
        histogram_storage.get(*args).observe(duration)
        ^^^^^^^^^^^^^^^^^^^^^
      AttributeError: 'NoneType' object has no attribute 'get'

To ensure that the searx.search module is initialized, the

- searx.engines.load_engines is replace by
- searx.search.initialize

[1] https://github.com/searxng/searxng/pull/3932#discussion_r1822406569

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

tests/unit/processors/test_online.py

@@ -3,7 +3,7 @@
 
 from searx.search import SearchQuery, EngineRef
 from searx.search.processors import online
-from searx.engines import load_engines
+import searx.search
 from searx import engines
 
 from tests import SearxTestCase
@@ -22,10 +22,10 @@ TEST_ENGINE = {
 class TestOnlineProcessor(SearxTestCase):  # pylint: disable=missing-class-docstring
 
     def setUp(self):
-        load_engines([TEST_ENGINE])
+        searx.search.initialize([TEST_ENGINE])
 
     def tearDown(self):
-        load_engines([])
+        searx.search.load_engines([])
 
     def _get_params(self, online_processor, search_query, engine_category):
         params = online_processor.get_params(search_query, engine_category)