Graph test

[djarecka]

Summary

• adding test for plotting graph for workflow when workflow has a splitter (test for changes from #538)
• small fix of Docker/Singularity tasks: name=None in init
• marking one test as flaky for slurm

Checklist

• [ ] I have added tests to cover my changes (if necessary)
• [ ] I have updated documentation (if necessary)

[codecov[bot]]

Codecov Report

Base: 76.82% // Head: 76.82% // No change to project coverage :thumbsup:

Coverage data is based on head (986d539) compared to base (a9ef35f). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #603 +/- ## ======================================= Coverage 76.82% 76.82% ======================================= Files 19 19 Lines 4358 4358 Branches 1258 1258 ======================================= Hits 3348 3348 Misses 821 821 Partials 189 189 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `76.73% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/nipype/pydra/pull/603?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [pydra/engine/task.py](https://codecov.io/gh/nipype/pydra/pull/603/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-cHlkcmEvZW5naW5lL3Rhc2sucHk=) | `86.76% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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