Improve IKEA Starkvind test

[TheJulianJES]

Regarding the issue fixed here (infinite recursion):

• Fix IKEA Starkvind attribute reports #125

It seems like we do have a unit test for this in test_fan.py, but it didn't catch the issue, as we're (1) not testing getting a fan_mode attribute report from the device and/or (2) because when fan_mode is changed from ZHA, we're only checking if the attribute was written correctly. Not if the entity was also updated correctly.
Doing either of those things would have caught the issue.

Further fixes and changes will come with #87 in the future.
We should also make sure the Starkvind test is improved for this scenario there (test speed) or shortly after that PR is merged.