[Modular] Some clean up for Modular tests

[DN6]

What does this PR do?

Some changes to make modular tests more in line with our new testing approach

1. Remove unittest dependency
2. Move get_pipeline to ModularPipelineTesterMixin (it's most likely going to be the same across all pipelines)
3. Create distinction between tests and test mixins. Since we don't use unittest anymore, we need to be explicit about which objects are Mixins and which objects are actually running the tests and need attributes such as param, repo etc.
4. Always use cpu based Generator through the self.get_generator method in ModularPipelineTesterMixin for better reproducibility (GPU based generator can introduce noise)
5. Use torch.abs().max for comparing slices. Let's move away from np conversions. There isn't much of an advantage and when we run on GPU we can avoid .detach().cpu().numpy() calls
6. Use single level inheritance for easier debugging.

Fixes # (issue)

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[sayakpaul]

(nit): torch.manual_seed(seed) is also enough.

[sayakpaul]

Very nice!