tests: drivers: adc_dma: fix sampling interval failing NXP tests

In 6e21ebf2 the sampling interval was changed, which caused the
issue #56070. This is fixed by allowing different boards to
specify a sampling period.

Also changed the test_task_with_interval to verify that
the supplied interval was used.

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>