Add unit test for Changes to cover get_source re-call

tests/unit/tools/debian/test_changes.py

+#   test_changes.py - unit testing for Changes
+#
+#   This file is part of debexpo
+#   https://salsa.debian.org/mentors.debian.net-team/debexpo
+#
+#   Copyright © 2019-2021 Baptiste Beauplat <lyknode@debian.org>
+#
+#   Permission is hereby granted, free of charge, to any person
+#   obtaining a copy of this software and associated documentation
+#   files (the "Software"), to deal in the Software without
+#   restriction, including without limitation the rights to use,
+#   copy, modify, merge, publish, distribute, sublicense, and/or sell
+#   copies of the Software, and to permit persons to whom the
+#   Software is furnished to do so, subject to the following
+#   conditions:
+#
+#   The above copyright notice and this permission notice shall be
+#   included in all copies or substantial portions of the Software.
+#
+#   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+#   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+#   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+#   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+#   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+#   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+#   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+#   OTHER DEALINGS IN THE SOFTWARE.
+
+from tests import TestController
+from os.path import join, dirname
+
+from debexpo.tools.debian.changes import Changes
+
+CHANGES_FILE_OK = join(dirname(__file__), '..', '..', '..', 'functional',
+                       'importer', 'data', 'ok', 'hello_1.0-1_amd64.changes')
+
+
+class TestChanges(TestController):
+    def test_dsc_from_changes(self):
+        changes = Changes(CHANGES_FILE_OK)
+        changes.validate()
+        changes.parse_dsc()
+
+        source = changes.get_source()
+        try:
+            source.extract()
+            source.parse_control_files()
+        finally:
+            source.remove()
+
+        self.assertEquals(source, changes.get_source())