ztest: Add test duration to summary for new ztest

Add duration stats at both suite level and unit test
level into test summary. The duration is at second
level. Since the new ztest fx can execute a test suite
for multiple times, the worst/longest test duration is
collected.

Note that even a skipped test can have a duration greater
than 0 because the skip operation itself is not free.

Signed-off-by: Ming Shao <ming.shao@intel.com>