tests: Separate lister instantiations

Prior to this commit, all listers were instantiated at the same time even if
only one was needed. This commit separates those instantiations.

The only drawback to this is the db model initialization which now happens at
each lister instantiation. This can be dealt with if needed at another time
though.

swh/lister/gitea/tests/test_lister.py

@@ -7,6 +7,8 @@ import logging
 import re
 import unittest
 
+import pytest
+
 from swh.lister.core.tests.test_lister import HttpListerTesterBase
 from swh.lister.gitea.lister import GiteaLister
 
@@ -38,8 +40,13 @@ class GiteaListerTester(HttpListerTesterBase, unittest.TestCase):
         return headers
 
 
-def test_lister_gitea(swh_listers, requests_mock_datadir):
-    lister: GiteaLister = swh_listers["gitea"]
+@pytest.fixture
+def lister_under_test():
+    return "gitea"
+
+
+def test_lister_gitea(swh_lister, requests_mock_datadir):
+    lister: GiteaLister = swh_lister
 
     lister.run()
     r = lister.scheduler.search_tasks(task_type="load-git")