Verify quality of test coverage

[tomato42]

Use mutation testing to verify if the existing test coverage is good enough.

Options:

• cosmic-ray (docs) (see also executing cosmic-ray for subset of code? sixty-north/cosmic-ray#484)
• mutmut

[tomato42]

given that we are able to verify something like 20 mutants in 25 minutes in Travis, we would be able to mutation test only the simplest of simple patches – unusable without #127, maybe usable with it

either way, not for 0.14

side note: now, just before 0.14 is released (d6cb288), the test suite has 23.93% survival rate (76.07% mutation score)

[tomato42]

with 0.16 we improved a bit, the survival rate is 21.33%

[tomato42]

with 0.18.0 we improved a lot, the survival rate is now 16.86%, though I executed the unchanged test suite (not the one with changes from #156) and cosmic-ray changed significantly too

[tomato42]

And before 0.19 release the survival rate is 21.51% (mutation score 78.49%), but we're able to execute about 33% of all mutations, so the confidence interval for survival rate is quite narrow: 20.80 to 22.22%.