[PATCH] [test-suite, LNT] Make the use of perf optional

[Yi Kong]

Hi all,

This patchset removes perf detection in timeit.sh and replace it with
an option in LNT, so that user can have more control about which tool
to use. Also this fixes my previous error which caused execution time
not actually being measured using perf.

Tobias, Renato: You will need to switch on "--use-perf" option in your
zorg config to enable perf on your performance builders.

Ok to commit?

Cheers,
Yi
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