Allow to specify duration per test

[artemrizhov]

I have a test that is working on large data set to ensure the programm is efficient on both big and small data. This test should be run for more than one second, while I'd not like to enlarge duration for all another tests.
Maybe something like module attribute would work for this.

@duration 5
bench "some long test:", do ...

[alco]

Hey @artemrizhov. I like the idea. Could you explain why this particular test has to run for a longer time?