banners/tests: Fix test failing with django 4.x

Related to #4734

swh/web/banners/tests/test_banners.py

@@ -67,6 +67,6 @@ def test_fundraising_banner_give_api_error(client, requests_mock):
 def test_downtime_banner(client):
     url = reverse("swh-downtime-banner")
     resp = check_html_get_response(
-        client, url, status_code=200, template_used="./downtime-banner.html"
+        client, url, status_code=200, template_used="downtime-banner.html"
     )
     assert_contains(resp, "Downtime expected for SWH services")

swh/web/banners/views.py

@@ -76,5 +76,5 @@ def downtime_banner(request):
     return render(
         request,
         "banner-iframe.html",
-        {"lang": lang if lang else "en", "banner_template": "./downtime-banner.html"},
+        {"lang": lang if lang else "en", "banner_template": "downtime-banner.html"},
     )