tests: Fix existing markers and enforce --strict-markers

pytest.ini

@@ -5,6 +5,7 @@ addopts =
     -p swh.web.tests.pytest_plugin
     --ignore-glob=*random_fixtures_test.py
     --ignore-glob=*create_test_*.py
+    --strict-markers
 norecursedirs = build docs node_modules resources .tox
 DJANGO_SETTINGS_MODULE = swh.web.settings.tests
 filterwarnings =
@@ -12,6 +13,7 @@ filterwarnings =
     ignore:.*Using or importing the ABCs from 'collections'
     ignore:.*uses the.*fixture, which is reset between function calls
     ignore:.*'U' mode is deprecated
-
 asyncio_mode = strict
 consider_namespace_packages = true
+markers =
+    inbound_message: used to pass a message parameter to the inbound_message fixture
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