Create test plot function

[mraffly]

Refers to point 3.1 in the following documentation: https://iffmd.fz-juelich.de/zXxTrardSNaJc4a22gNypw?view

Create test plot functions for two test functions which so fare are still marked as "skipped". See the following lines of codes:

https://github.com/FZJ-IEK3-VSA/FINE/blob/c28d5b87983016af57680ae69adf55defb36b4dc/test/IOManagement/test_plot.py#L10-12

https://github.com/FZJ-IEK3-VSA/FINE/blob/c28d5b87983016af57680ae69adf55defb36b4dc/test/IOManagement/test_plot.py#L15-17