Indicate freshness of test run

[lread]

Hey there, I am really enjoying using cljs-test-display via figwheel-main, thanks for creating these great tools!

Some indicator of test freshness in the test summary would be of interest to me. I am writing macros and as such am restarting my repl quite a bit and sometimes I don't realize that my test run is stale.

I am think of something along the lines of these variants:

• Last run less than a minute ago
• Last run 1 minute ago
• Last run 13 minutes ago
• Last run 1 day ago

I think a good spot might be under the Totals, so something like:

Totals
190 Tests 1490 Assertions
Last run 3 days ago

If you think this idea is makes sense and is interesting, let me know, I am happy to work on the pull request.