dmatest: replace stored results mechanism, with uniform messages

For long running tests the tracking results in a memory leak for the "ok"
results, and for the failures the kernel log should be sufficient.  Provide a
uniform format for error messages so they can be easily parsed and remove the
debugfs file.

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