- add `static` qualifiers where sensible
- drop `volatile` qualifier that was incorrectly used
- add a comment on use of shared memory for inter-thread communication
19565: tests: move core related applications to their own tests/core/ folder r=maribu a=aabadie
19568: tests: move remaining driver related applications to tests/drivers r=maribu a=aabadie
19574: cpu/stm32/periph_gpio: reset PU/PD for ADC channels r=maribu a=gschorcht
### Contribution description
This PR provides a small fix that is relevant when a GPIO has been used as input/output with a pull resistor before it is initialized as an ADC channel.
The PU/PD configuration has to be `0b00` for analog outputs which is corresponds to the reset state. However, if the GPIO is not in the reset state but was used digital input/output with any pull resistor, the PU/PD configuration has also to be reset to use it as ADC channel.
### Testing procedure
- Green CI
- The `periph_adc` test application should still work for any board that supports the `periph_adc` feature.
### Issues/PRs references
Co-authored-by: Alexandre Abadie <alexandre.abadie@inria.fr>
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
