dm: Rename DM test flags to make them more generic

dm: Rename DM test flags to make them more generic

The test flags used by driver model are currently not available to other
tests. Rather than creating two sets of flags, make these flags generic
by changing the DM_ prefix to UT_ and moving them to the test.h header.

This will allow adding other test flags without confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>