test: Handle driver model reinit in test_pre_run()

For driver model tests we want to reinit the data structures so that
everything is in a known state before the test runs. This avoids one test
changing something that breaks a subsequent tests.

Move the call for this into test_pre_run().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/test-main.c b/test/test-main.c
index 6f0d32f..f14b7b0 100644
--- a/test/test-main.c
+++ b/test/test-main.c
@@ -30,6 +30,9 @@
 
 int test_pre_run(struct unit_test_state *uts, struct unit_test *test)
 {
+	if (test->flags & UT_TESTF_DM)
+		ut_assertok(dm_test_init(uts));
+
 	ut_set_skip_delays(uts, false);
 
 	uts->start = mallinfo();