show test run overview on completion

[boegel]

Currently when a test run completes, reframe doesn't give much information, just a single line like:

[  PASSED  ] Ran 717 test case(s) from 717 check(s) (0 failure(s))
[==========] Finished on Sat Dec  7 14:54:30 2019

I think it could be useful to provide more information, like:

• start time, end time, total time that was required
• avg/min/max/median duration of tests
• where to find test outputs

[vkarak]

Hi @boegel, this is related to #97 and it is in our roadmap.