Improve the testing suite for FuncX

[WardLT]

The FuncXExecutor has some behavior around threads we don't mock, but that caused the FuncXTaskServer to break when FuncX updated to v1. We should make our mock executor wrap the FuncXExecutor (and, perhaps, mark the futures as complete once _batch_run returns them) so that we ensure that Colmena is properly tested against FuncX

See e5dbc04 for details