18632: tests/thread_float: do not overload slow MCUs with IRQs r=kaspar030 a=maribu

### Contribution description

If the regular context switches are triggered too fast, slow MCUs will be able to spent little time on actually progressing in the test. This will scale the IRQ rate with the CPU clock as a crude way too keep load within limits.

### Testing procedure

The unit test should now pass on the Microduino CoreRF

```
$ make BOARD=microduino-corerf AVRDUDE_PROGRAMMER=dragon_jtag -C tests/thread_float flash test
make: Entering directory '/home/maribu/Repos/software/RIOT/tests/thread_float'
Building application "tests_thread_float" for "microduino-corerf" with MCU "atmega128rfa1".
[...]
   text	  data	   bss	   dec	   hex	filename
  12834	   520	  3003	 16357	  3fe5	/home/maribu/Repos/software/RIOT/tests/thread_float/bin/microduino-corerf/tests_thread_float.elf
avrdude -c dragon_jtag -p m128rfa1  -U flash:w:/home/maribu/Repos/software/RIOT/tests/thread_float/bin/microduino-corerf/tests_thread_float.hex
[...]
Welcome to pyterm!
Type '/exit' to exit.
READY
s
START
main(): This is RIOT! (Version: 2022.10-devel-858-g18566-tests/thread_float)
THREADS CREATED

Context switch every 3125 µs
{ "threads": [{ "name": "idle", "stack_size": 192, "stack_used": 88 }]}
{ "threads": [{ "name": "main", "stack_size": 640, "stack_used": 220 }]}
THREAD t1 start
THREAD t2 start
THREAD t3 start
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770
t3: 141.466810
t1: 141.443770

make: Leaving directory '/home/maribu/Repos/software/RIOT/tests/thread_float'
```

(~~Note: The idle thread exiting is something that should never occur. I guess the culprit may be `cpu_switch_context_exit()` messing things up when the main thread exits. But that is not directly related to what this PR aims to fix. Adding a `thread_sleep()` at the end of `main()` does indeed prevent the idle thread from exiting.~~
Update: That's expected. The idle thread stats are printed on exit of the main thread, the idle thread does not actually exit.)

### Issues/PRs references

Fixes #16908 maybe?

19031: cpu/stm32/periph_timer: implement timer_set() r=benpicco a=maribu

### Contribution description

The fallback implementation of timer_set() in `drivers/periph_common` is known to fail on short relative sets. This adds a robust implementation.

### Testing procedure

Run `tests/periph_timer_short_relative_set` at least a few dozen times (or use #19030 to have a few dozen repetitions of the test case in a single run of the test application). It should now succeed.

### Issues/PRs references

None

Co-authored-by: Marian Buschsieweke <[email protected]>