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/test/Kconfig b/test/Kconfig
index 558a9cd..1c8e3d1 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -20,6 +20,15 @@
 	  of-platdata and SPL handover. To run these tests with the sandbox_spl
 	  board, use the -u (unit test) option.
 
+config UNIT_TEST_DURATION
+	bool "Report unit-test duration"
+	depends on UNIT_TEST
+	default y
+	help
+	  Enable this short the time taken by each test suite. This is reported
+	  after the suite runs, alongside the pass/fail results. In addition,
+	  an overall total is reported if multiple suites are run.
+
 config UT_LIB
 	bool "Unit tests for library functions"
 	depends on UNIT_TEST