Replace operator == for floating-point comparison in test

[daniel-zullo-frequenz]

What's needed?

Replace operator == in https://github.com/frequenz-floss/frequenz-sdk-python/blob/v1.x.x/tests/actor/test_data_sourcing.py to compare floating-point values against the result of addition operations involving floating-point numbers.

Proposed solution

Use isclose() from pytest or math instead.

Additional context

This is a follow-up from a code review: https://github.com/frequenz-floss/frequenz-sdk-python/pull/955/files#r1630265422

[llucax]

These tests might need to be changed when upgrading to the microgrid API v0.17, so maybe it can be addressed then.