tests: fix test_find_path_for_payment. need to close sqlite connection

test was sometimes randomly failing
(always on Windows, as it's illegal to rm open files there)

electrum/sql_db.py

@@ -54,4 +54,5 @@ class SqlDB(Logger):
                     self.conn.commit()
         # write
         self.conn.commit()
+        self.conn.close()
         self.logger.info("SQL thread terminated")

electrum/tests/test_lnrouter.py

@@ -105,7 +105,10 @@ class Test_LNRouter(TestCaseForTestnet):
         self.assertEqual(route[0].node_id, start_node)
         self.assertEqual(route[0].short_channel_id, bfh('0000000000000003'))
 
-
+        # need to duplicate tear_down here, as we also need to wait for the sql thread to stop
+        self.asyncio_loop.call_soon_threadsafe(self._stop_loop.set_result, 1)
+        self._loop_thread.join(timeout=1)
+        cdb.sql_thread.join(timeout=1)
 
     def test_new_onion_packet(self):
         # test vector from bolt-04