[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/test_results.py

@@ -2,7 +2,7 @@
 # pylint: disable=missing-module-docstring
 
 from searx.results import ResultContainer
-from searx.engines import load_engines
+import searx.search
 from tests import SearxTestCase
 
 
@@ -36,17 +36,16 @@ def fake_result(url='https://aa.bb/cc?dd=ee#ff', title='aaa', content='bbb', eng
 
 
 class ResultContainerTestCase(SearxTestCase):  # pylint: disable=missing-class-docstring
+
     def setUp(self) -> None:
         stract_engine = make_test_engine_dict(name="stract", engine="stract", shortcut="stra")
         duckduckgo_engine = make_test_engine_dict(name="duckduckgo", engine="duckduckgo", shortcut="ddg")
         mojeek_engine = make_test_engine_dict(name="mojeek", engine="mojeek", shortcut="mjk")
-
-        load_engines([stract_engine, duckduckgo_engine, mojeek_engine])
-
+        searx.search.initialize([stract_engine, duckduckgo_engine, mojeek_engine])
         self.container = ResultContainer()
 
     def tearDown(self):
-        load_engines([])
+        searx.search.load_engines([])
 
     def test_empty(self):
         self.assertEqual(self.container.get_ordered_results(), [])