Harmonize tests across ttsim/gettsim

[hmgaudecker]

In #880, we updated src/_gettsim_tests/utils.py but missed tests/ttsim/utils.py, which will be adjusted in #876. Most of the code is duplicated in the two files.

Solution

• Add a file test_utils or so to ttsim, which can be used by both. Main issue preventing this ATM is the usage of globals, which we will need to get rid of, anyhow.
• As a bonus, convert the qualified names in the output dataframes to a MultiIndex (unless MultiIndex.from_tuples works out of the box, I guess the trick will be to first determine the maximum level of nesting and then pad shorter tuples with empty elements)

[hmgaudecker]

Fixed in #948.