test: Keep track of suite duration

Show the time taken by each test suite with 'ut all' and the total time
for all suites.

Take care to remove any sandbox time-offset from the values.

Fix the comment-format on timer_test_add_offset() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/test/test.h b/include/test/test.h
index bac43c8..25c7712 100644
--- a/include/test/test.h
+++ b/include/test/test.h
@@ -16,11 +16,15 @@
  * @skip_count: Number of tests that were skipped
  * @test_count: Number of tests run. If a test is run muiltiple times, only one
  *	is counted
+ * @start: Timer value when test started
+ * @duration_ms: Suite duration in milliseconds
  */
 struct ut_stats {
 	int fail_count;
 	int skip_count;
 	int test_count;
+	ulong start;
+	ulong duration_ms;
 };
 
 /*