test/dm: clear unit test failure count each run

The ut command prints a test failure count each time it is executed.
This is stored in a global variable which is never reset. Consequently,
the printed failure count accumulates across runs. Fix this by clearing
the counter each time "ut" is invoked.
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Acked-by: NSimon Glass <sjg@chromium.org>

test/dm/test-main.c

@@ -81,6 +81,8 @@ static int dm_test_main(const char *test_name)
 	struct unit_test *test;
 	int run_count;
 
+	uts->fail_count = 0;
+
 	/*
 	 * If we have no device tree, or it only has a root node, then these
 	 * tests clearly aren't going to work...