test: Show the average time per test

test: Show the average time per test

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>