Test issues windows

[h-bode]

Running Pytest with windows generates an error. The second tests of the runner causes an assertion error.
Running the same file in Linux doesn´t cause problems.

Idea was that the fmu might be different. The test.fmu which is used for linux is the grid.network from april 20th, 2020.

I restored the grid.mo of this date, created a new test.fmu, and ran it, same error as the original one. Daniel did the same on his Linux, no errors.

And now the interesting part: using the actual grid.mo from the dev to create the test.fmu causes 5 errors (runner and modelica) ob windows, doing the same on Daniels linux machine doesn´t cause an error at all.

The differences between the OS seem to be larger than we expected...

[stheid]

related to

#115
#113

[wallscheid]

additional headline in readme to highlight how to execute the pytests plus comment on windows-related issue (with link)