fix: add postgres.dump_db() function

tests/test_postgres.py

@@ -122,3 +122,13 @@ class TestDropDb(TestCase):
             mod.drop_db(c, 'foo')
             db_exists.assert_called_once_with(c, 'foo')
             c.sudo.assert_not_called()
+
+
+class TestDumpDb(TestCase):
+
+    def test_basic(self):
+        c = MagicMock()
+        result = mod.dump_db(c, 'foo')
+        self.assertEqual(result, 'foo.sql.gz')
+        c.sudo.assert_called_once_with('pg_dump foo | gzip -c > /tmp/foo.sql.gz', user='postgres')
+        c.run.assert_called_with('rm /tmp/foo.sql.gz')