test: Reduce the number of tests run with sandbox_flattree

We only need to run driver-model tests with this config, since this is the
only thing that is different when CONFIG_OF_LIVE is not defined. Filter
out the other tests to same time.

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

test/run

@@ -25,7 +25,8 @@ run_test "sandbox_spl" ./test/py/test.py --bd sandbox_spl --build \
 # build which does not enable CONFIG_OF_LIVE for the live device tree, so we can
 # check that functionality is the same. The standard sandbox build (above) uses
 # CONFIG_OF_LIVE.
-run_test "sandbox_flattree" ./test/py/test.py --bd sandbox_flattree --build
+run_test "sandbox_flattree" ./test/py/test.py --bd sandbox_flattree --build \
+	-k test_ut
 
 # Set up a path to dtc (device-tree compiler) and libfdt.py, a library it
 # provides and which is built by the sandbox_spl config.