Add testcases

[Sh3Rm4n]

For current bugs:

• [ ] #109
• [ ] #54
• [x] #125 (3.)
  • Add a test, which checks, that the pull state is correctly reset, when transitioning the pin type state.

and definitely more:

• [x] adc
• [ ] delay
• [x] can (example and after #207 bxcan is well testsed)
• [ ] dma (partially covered by example)
• [ ] ~~flash~~ (not really needed)
• [x] gpio
• [ ] i2c (partially by example)
• [ ] pwm
• [x] rcc
• [ ] rtc
• [x] serial
• [x] spi
• [ ] ~~syscfg~~ (not really needed)
• [x] timer
• [ ] usb
• [x] watchdog

[Sh3Rm4n]

This is worth a read, to improve our test coverage: https://ferrous-systems.com/blog/defmt-test-hal/