test: added test coverage to loadtxoutset

The functional test coverage did not cover the rpc error of Couldn't
open file for loadtxoutset and this test adds coverage for it

test/functional/feature_assumeutxo.py

@@ -152,6 +152,12 @@ def test_invalid_mempool_state(self, dump_output_path):
 
         self.restart_node(2, extra_args=self.extra_args[2])
 
+    def test_invalid_file_path(self):
+        self.log.info("Test bitcoind should fail when file path is invalid.")
+        node = self.nodes[0]
+        path = node.datadir_path / node.chain / "invalid" / "path"
+        assert_raises_rpc_error(-8, "Couldn't open file {} for reading.".format(path), node.loadtxoutset, path)
+
     def run_test(self):
         """
         Bring up two (disconnected) nodes, mine some new blocks on the first,
@@ -236,6 +242,7 @@ def run_test(self):
         self.test_invalid_mempool_state(dump_output['path'])
         self.test_invalid_snapshot_scenarios(dump_output['path'])
         self.test_invalid_chainstate_scenarios()
+        self.test_invalid_file_path()
 
         self.log.info(f"Loading snapshot into second node from {dump_output['path']}")
         loaded = n1.loadtxoutset(dump_output['path'])