Need (optional) parameter for test duration

[afbjorklund]

When passing in a pre-generated json file, the duration is for reading the file

$ gotestsum --jsonfile test_report.json
✓  . (8ms)
∅  embed_assets

DONE 16 tests in 0.569s
$ go-test-report < test_report.json 
[go-test-report] finished in 115.295321ms

⧉ Total: 16 Duration: 1ms ✓ Passed: 16 ‐ Skipped: 0 ✗ Failed: 0

If you want to show the original time it took to run the test, it needs a param

--duration 0.569

perhaps parse any "s" or "ms" suffix in the parameter, for user convenience.