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/bitbucket/tests/test_lister.py

@@ -1,4 +1,4 @@
-# Copyright (C) 2017-2019 The Software Heritage developers
+# Copyright (C) 2017-2020 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -10,12 +10,18 @@ from datetime import timedelta
 from urllib.parse import unquote
 
 import iso8601
+import pytest
 import requests_mock
 
 from swh.lister.bitbucket.lister import BitBucketLister
 from swh.lister.core.tests.test_lister import HttpListerTester
 
 
+@pytest.fixture
+def lister_under_test():
+    return "bitbucket"
+
+
 def _convert_type(req_index):
     """Convert the req_index to its right type according to the model's
        "indexable" column.
@@ -80,11 +86,11 @@ class BitBucketListerTester(HttpListerTester, unittest.TestCase):
         )
 
 
-def test_lister_bitbucket(swh_listers, requests_mock_datadir):
+def test_lister_bitbucket(swh_lister, requests_mock_datadir):
     """Simple bitbucket listing should create scheduled tasks (git, hg)
 
     """
-    lister = swh_listers["bitbucket"]
+    lister = swh_lister
 
     lister.run()