package.tests: use a fixture to configure the loaders

This allows to make test execution not depend on the SWH_CONFIG_FILENAME
environment variable, so it's easier to run them using pytest directly
(rather than tox).

swh/loader/package/tests/conftest.py

+import os
+import pytest
+
+from .common import DATADIR
+
+
+@pytest.fixture
+def swh_config(monkeypatch):
+    conffile = os.path.join(DATADIR, 'loader.yml')
+    monkeypatch.setenv('SWH_CONFIG_FILENAME', conffile)
+    return conffile

swh/loader/package/tests/resources/loader.yml

+storage:
+  cls: memory
+  args: {}

swh/loader/package/tests/test_gnu.py

@@ -132,7 +132,7 @@ _expected_branches_first_visit = {
 _expected_new_snapshot_first_visit_id = 'c419397fd912039825ebdbea378bc6283f006bf5'  # noqa
 
 
-def test_release_artifact_not_found(requests_mock):
+def test_release_artifact_not_found(swh_config, requests_mock):
     package = '8sync'
     package_url = 'https://ftp.gnu.org/gnu/8sync/'
     tarballs = [{
@@ -161,7 +161,7 @@ def test_release_artifact_not_found(requests_mock):
     } == stats
 
 
-def test_release_artifact_no_prior_visit(requests_mock):
+def test_release_artifact_no_prior_visit(swh_config, requests_mock):
     """With no prior visit, load a pypi project ends up with 1 snapshot
 
     """

swh/loader/package/tests/test_npm.py

@@ -432,7 +432,7 @@ def package_metadata_url(package):
     return 'https://replicate.npmjs.com/%s/' % package
 
 
-def test_npm_loader_2_first_visit(requests_mock):
+def test_npm_loader_2_first_visit(swh_config, requests_mock):
 
     package = 'org'
     requests_mock.get(re.compile('https://'),

swh/loader/package/tests/test_pypi.py

@@ -124,7 +124,7 @@ def test_author_malformed_3():
 
 def test_badly_configured_loader_raise(monkeypatch):
     """Badly configured loader should raise"""
-    monkeypatch.delenv('SWH_CONFIG_FILENAME')
+    monkeypatch.delenv('SWH_CONFIG_FILENAME', raising=False)
     with pytest.raises(ValueError) as e:
         PyPILoader(url='some-url')
 
@@ -225,12 +225,10 @@ def test_no_release_artifact(requests_mock):
 
 # "normal" cases (for the same origin) #
 
-def test_release_artifact_no_prior_visit(requests_mock):
+def test_release_artifact_no_prior_visit(swh_config, requests_mock):
     """With no prior visit, load a pypi project ends up with 1 snapshot
 
     """
-    assert 'SWH_CONFIG_FILENAME' in os.environ  # cf. tox.ini
-
     loader = PyPILoader('https://pypi.org/project/0805nexter')
     requests_mock.get(re.compile('https://'), body=get_response_cb)