exekall: pretty print test metrics

Finally get some decent output, such as this exekall summary:

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[EXEKALL][2019-10-21 18:32:55,548] INFO  Artifacts dir: /home/dourai01/Data/Git/lisa/results/pwrsft-juno-r0-1/20191021_18:32:52_2474a9215fc54e2ca141b6147bfd3da4
[EXEKALL][2019-10-21 18:32:55,548] INFO  Result summary:
OneSmallTask[board=pwrsft-juno-r0-1]:test_dmesg          UUID=968577749bcb4aea817a593d6a3f4ad7 PASSED: dmesg output:
OneSmallTask[board=pwrsft-juno-r0-1]:test_slack          UUID=c92c7d5ae0564db1bab80d12e25f362c PASSED: small-0 delayed activations: 0.0 %
OneSmallTask[board=pwrsft-juno-r0-1]:test_task_placement UUID=1bc2c5fc904f43e58f7f2bfb5754a271 PASSED
    energy threshold: 40.16330044843049 bogo-joules
    estimated energy: 37.66641002577397 bogo-joules
    noisiest task: 
        comm: rcu_preempt
        duration (abs): 0.00030419999984587776 s
        duration (rel): 0.030686161314524632 %
        pid: 10

note: When not nested, a single line is used. When more items are available, the multiline output is used automatically. note2: The ResultBundle's UUID is now displayed, so its artifacts can be easily looked up in <artifact dir>/BY_UUID/<UUID>