tests: drivers: uart: uart_mix_fifo_poll: Move counter handling

Move enabling of counter to the test instead of test setup. Test may be
skipped in some configurations and in that case counter shall not be
started so by moving setup to the test code allows skipping test before
counter is started.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>