tests: test_dma_loop: Release DMA channels upon use

Release DMA channels allocated through dma_request_channel with
dma_release_channel in test_loop, test_loop_suspend_resume and
test_loop_repeated_start_stop.

This prevents channel allocation leaks and enables the tests to still
run even if the utilised DMA driver is configured to only utilise a
subset of implemented DMA channels.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>