Verify that SDF-related functionality is compatible with multiworker environment for FnApiRunner

The FnApiRunner currently avoids splitting deferred inputs for multiple workers: [https://github.com/apache/beam/blob/release-2.16.0/sdks/python/apache_beam/runners/portability/fn_api_runner.py#L771-L793](https://github.com/apache/beam/blob/release-2.16.0/sdks/python/apache_beam/runners/portability/fn_api_runner.py#L771-L793)

These issues are being surfaced as I convert the FnApiRunner to work based on ready elements instead of executing stage per stage: [https://github.com/apache/beam/pull/10067](https://github.com/apache/beam/pull/10067)

We should verify that the work items coming back from parallel SDFs are being merged properly.

Symptoms that I'm seeing are duplication of element processing for SDF tests:
```

TEST: apache_beam.runners.portability.fn_api_runner_test.FnApiRunnerSplitTestWithMultiWorkers.test_split_crazy_sdf
EXPECTED:
[

(5, 0), (5, 1), (5, 2), (5, 3), (5, 4),
(9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9,
7), (9, 8),
(8, 0), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7),
(8, 0), (8, 1), (8,
2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7),
(9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6),
(9, 7), (9, 8)] ==

ACTUAL OUTPUT:
[
(8, 0), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8,
7),
(9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8),
(8, 0), (8, 1), (8,
2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7),
(9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 7), (9, 8),
(9, 5), (9, 6),
(5, 0), (5, 1), (5, 2), (5, 3), (5, 4),
(8, 0), (8, 1), (8, 2), (8, 3), (8, 4),
(8, 5), (8, 6), (8, 7),
(9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8)]

```

Some comments from Robert:

 
```

Or the _add_residuals_and_channel_splits_to_deferred_inputs method. Looks like it has side effects?Hmm....
looks like this code was added for the multi-worker case. (And the comments and the TODO are unrelated.)I
think this is in reference to the fact that https://github.com/apache/beam/blob/master/sdks/python/apache_beam/runners/portability/fn_api_runner.py#L1826 does
not do the right thing yet, but I wonder how that's OK the first time around....there might be a bug
lurking here. 

```

cc: [~boyuanz], [~robertwb]

Imported from Jira [BEAM-8833](https://issues.apache.org/jira/browse/BEAM-8833). Original Jira may contain additional context.
Reported by: pabloem.