Fix synapse.config module "read" command (#11145)

`synapse.config.__main__` has the possibility to read a config item. This can be used to conveniently also validate the config is valid before trying to start Synapse.

 The "read" command broke in https://github.com/matrix-org/synapse/pull/10916 as it now requires passing in "server.server_name" for example.

 Also made the read command optional so one can just call this with just the confirm file reference and get a "Config parses OK" if things are ok.

Signed-off-by: Jason Robinson <jasonr@matrix.org>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

tests/config/test___main__.py

@@ -0,0 +1,31 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from synapse.config.__main__ import main
+
+from tests.config.utils import ConfigFileTestCase
+
+
+class ConfigMainFileTestCase(ConfigFileTestCase):
+    def test_executes_without_an_action(self):
+        self.generate_config()
+        main(["", "-c", self.config_file])
+
+    def test_read__error_if_key_not_found(self):
+        self.generate_config()
+        with self.assertRaises(SystemExit):
+            main(["", "read", "foo.bar.hello", "-c", self.config_file])
+
+    def test_read__passes_if_key_found(self):
+        self.generate_config()
+        main(["", "read", "server.server_name", "-c", self.config_file])