tests: document the freq argument in test_set_event()
tests: document the freq argument in test_set_event()

The freq argument is not documented and it's not clear what it means.
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Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod