test/dma: add more comprehensive copy tests

test/dma: add more comprehensive copy tests

Add unit tests for various combinations of use for dmadev, copying
bursts of packets in various formats, e.g.

1. enqueuing two smaller bursts and completing them as one burst
2. enqueuing one burst and gathering completions in smaller bursts
3. using completed_status() function to gather completions rather than
   just completed()

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Kevin Laatz <kevin.laatz@intel.com>
Reviewed-by: Conor Walsh <conor.walsh@intel.com>