Mark tests needing TOML-writing library with a custom marker

This way they can be conveniently skipped with `pytest -m "not
tomlwrite"` in environments where such libraries aren't available.

stages/test/test_bootc_install_config.py

@@ -73,6 +73,7 @@ STAGE_NAME = "org.osbuild.bootc.install.config"
         )
     ]
 )
+@pytest.mark.tomlwrite
 def test_bootc_install_config(tmp_path, stage_module, options, expected_config):
     stage_module.main(tmp_path, options)
     # different toml libraries write out arrays in different ways (one line vs many), so we need to parse the result

stages/test/test_containers_storage_conf.py

@@ -18,6 +18,7 @@ TEST_INPUT = [
 STAGE_NAME = "org.osbuild.containers.storage.conf"
 
 
+@pytest.mark.tomlwrite
 @pytest.mark.parametrize("test_filename", ["/etc/containers/storage.conf", "/usr/share/containers/storage.conf"])
 @pytest.mark.parametrize("test_storage,test_options,expected", TEST_INPUT)
 def test_containers_storage_conf_integration(tmp_path, stage_module, test_filename, test_storage, test_options, expected):  # pylint: disable=unused-argument