Change testing to use new testing from Linopy v0.5.7

[FBumann]

Prerequisite Checks

• I have searched the existing issues and confirmed this feature doesn't already exist.

• I have checked the documentation to ensure this feature isn't already available.

Is your feature request related to a problem?

No.

Describe the solution you'd like

Update the testing from our own assert_conequal to linopys native assert_conequal (which is equal to ours, as I updated it)

Energy System Use Case

Only relevant for our internal testing

Desired Code Example (optional)

Component Category (if applicable)

None

Alternatives considered

No response

Additional context

No response