Show the average duration of a test, so we can keep track of how it is
trending. Report the suite with the longest average test to encourage
people to improve it.
Add a function to update the stats based on the results from a single
suite and another to show the summary information.
Make this optional, since sandbox's SPL tests do not have a timer driver
and people may want to print results without times.
Signed-off-by: Simon Glass <sjg@chromium.org>
835ede55fd