19587: tests/README.md: Add directory overview r=aabadie a=bergzand
### Contribution description
This adds a small overview of the different subdirectories now in the tests directory
### Testing procedure
- Read the new documentation
- Check the links
- Check whether I missed any new subdirectory in the list.
### Issues/PRs references
Related to the excellent cleanup work started by `@Teufelchen1` and `@aabadie`
Co-authored-by: Koen Zandberg <koen@bergzand.net>
19585: tests: move leftover sys related tests to test/sys + move tests/sys/candev to tests/drivers r=aabadie a=aabadie
19586: tests: move net related applications to test/net r=aabadie a=aabadie
Co-authored-by: Alexandre Abadie <alexandre.abadie@inria.fr>
19582: tests: move nimble and lwip related to test applications to tests/pkg r=aabadie a=aabadie
Co-authored-by: Alexandre Abadie <alexandre.abadie@inria.fr>
19573: cpu/stm32/periph_dac: small improvements r=maribu a=gschorcht
### Contribution description
This PR provides the following improvements for `periph_dac` on STM32
- Support for `RCC_APB1ENR1_DAC1EN` symbol added.
- For boards that have not connected the V_REF+ pin to an external reference voltage, the VREFBUF peripheral can be used as V_REF+ (if supported) by setting `VREFBUF_ENABLE=1`.
- If the DAC peripheral has a mode register (`DAC_MCR`), it is set to normal mode with buffer enabled and connected to external pin and on-chip peripherals. This allows to measure the current value of a DAC channel with an ADC channel or to use the DAC channel also for other on-chip peripherals.
### Testing procedure
- Green CI
- `tests/periph_dac` should still work for any board supporting the `periph_dac` feature.
### Issues/PRs references
19579: doc/doxygen/src/flashing.md: work around Doxygen bug r=maribu a=maribu
### Contribution description
Doxygen fails to render inline code in headers correctly in the version the CI uses. So, work around the issue by not typestetting `stm32flash` as inline code but as regular text.
19583: tests: move cpu related applications to tests/cpu r=maribu a=aabadie
19584: tests/build_system/external_board_dirs: fix broken symlinks r=maribu a=aabadie
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Co-authored-by: Alexandre Abadie <alexandre.abadie@inria.fr>
