Fix/issue 440 create compreh tests

[zljubisic]

I have added few tests to improve testing.
Feel free to add or change anything.
Closes: #440
@fboerman can you please check?

[Shohail-Ismail]

I was going to suggest some more tests for the repo (after adding them for a PR using EntsoePandasClient over at OpenClimateFix), before a search brought me here - where I can see that the proposed suite of tests is already quite comprehensive.

Therefore, @zljubisic I'd appreciate it if you could provide an update on the status of this PR, so I can open an Issue to add my tests if needed. Thanks.

[zljubisic]

Hi @Shohail-Ismail. I have created these tests long time ago. At that time some other guy also created PR with some tests.
I believe that tests from both PRs could be integrated and merged to the main branch.
If you want me to do something with this please let me know.

[Shohail-Ismail]

Thanks @zljubisic, I've opened an Issue related to the specific tests I wanted to add - will let you know if there's anything further related to your tests.