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: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-main.c b/test/dm/test-main.c
index 91bdda8..f2e0048 100644
--- a/test/dm/test-main.c
+++ b/test/dm/test-main.c
@@ -81,6 +81,8 @@
 	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...