test: Add ut_run_test_live_flat() to run tests twice

test: Add ut_run_test_live_flat() to run tests twice

Driver model tests are generally run twice, once with livetree enable and
again with it disabled. Add a function to handle this and call it from the
driver model test runner.

Make ut_run_test() private since it is not used outside test-main.c now.

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