dmatest: support xor-only, or pq-only channels in tests

Currently we only test raid channels that happen to also have 'copy'
capability.  Search for capable channels that do not have DMA_MEMCPY.

Note the return value from run_threaded_test never really made sense
because it could return errors after successfully starting tests.  We
already have the test results per channel so missing channels can be
detected at that time.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>