Sliding Sync: Split and move tests

changelog.d/17504.misc

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+Split and move Sliding Sync tests so we have some more sane test file sizes.

tests/rest/client/sliding_sync/__init__.py

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+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright 2015, 2016 OpenMarket Ltd
+# Copyright (C) 2023 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+# Originally licensed under the Apache License, Version 2.0:
+# <http://www.apache.org/licenses/LICENSE-2.0>.
+#
+# [This file includes modifications made by New Vector Limited]
+#
+#