tests: boards: nrf: i2s: i2s_divider: Confirm CLK divider value

Frame rate selected by the user may not be obtained due to
PCLK frequency and finite number of clock divider values.
In such case, driver shall select divider that results in
frame clock rate closest to the requested value.
There was a bug that was discarding perfect match divider.

Add test which confirms correct clock divider selection.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>