tests: pwm: led: Fix pwm_led sample

The PR fixes the pwm_led sample:
- now test doesn't rely on any specific device name
- Logs which are scanned with regex in the test are printed
  only after a command passes (before failures were not
  affecting the test verdict)
- If 1 sec cycle is not supported appropriate info is printed
  and won't cause the test to failed
- Changed second "Turned off" msg so regex doesn't mix it with the
  the first one.

Fixes #35524

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>