Simplify some ddp-spawn tests

[awaelchli]

What does this PR do?

Part of #10896

Some of these tests survived history and are written in a way that does not reflect anymore our new way of testing plugins.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @justusschock @awaelchli @akihironitta @Borda @kaushikb11

[Borda]

nice explanation :]