Add better test functions

# Feature
## Desired Behavior / Functionality
Current test functions are based only on polynomials and sinusoidal and exponential functions.

## What Needs to Be Done
- [ ] Find nice test functions (e.g. in [VEGAS Enhanced paper](https://arxiv.org/abs/2009.05112) )
- [ ] Add them to integration_test_functions.py
- [ ] Add them in integration_test_utils.py so they become part of the standard test sweet
- [ ] If necessary adjust current tests in case they fail

## How Can It Be Tested
- Run pytest

Read integration_test_functions.py and integration_test_utils.py, then review the linked VEGAS Enhanced paper for candidate test functions. Run pytest to establish the current baseline, add suitable functions and register them in the standard test suite, and adjust affected tests until pytest passes.

Written by the indexing model from the issue text.