[tests] component manager tests

[sayakpaul]

What does this PR do?

Adds tests for the component manager in modular pipelines.

• Correctness-related checks (offloading effectivity, component eviction, etc.). These tests are in ComponentsManagerTesterMixin.
• Checks when the component manager is used in actual modular pipelines. These are in ModularPipelineOffloadTesterMixin. I have only used it in the Flux pipeline tests to keep things minimal.

Notes:

We're mocking the available memory and free memory in the tests and checking against those simulations. So, in many cases we don't actually need a real GPU or a GPU with a certain amount of memory.

In the next PR, I want to introduce the maximum_total_models_size flag (and related tests).

[github-actions]

Hi @sayakpaul, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.