Explicit tests for tf_utils and qt_utils

[GlaserN]

Is your feature request related to a problem? Please describe.
We need explicit tests and extensive tests for the functions in tf_utils and qt_utils. Code coverage is not given by the current tests and several functions are not curretly working, correctly, e.g. tf_superoper_average_fidelity.
https://github.com/q-optimize/c3/blob/dev/c3/utils/tf_utils.py#L802-L807
As those functions are the backbone for all calculations good tests would be essential for all computations.

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Write tests

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Additional context

[lazyoracle]

Some tests for tf_utils functions were added in #99