stack test --coverage shouldn't include tests in reports

[mboes]

The intention of stack test --coverage is to provide a report about test coverage, i.e. how good are tests at exercising all of the library/program. In this light, we don't care about whether tests themselves have good coverage. Typically tests have very good coverage, but programs very bad coverage. So including the tests in the results biases the overall coverage report towards being too optimistic.

Perhaps adding/removing tests from the coverage report should be the matter of a flag.

[mgsloan]

Another aspect of this is that currently coverage applies to all of the packages at once. This is pretty cool, but probably not what you want as a default. With larger projects it also seems to be prone to triggering some ghc / hpc bugs (#607).

Thankfully, hpc has an --include flag which allows you to restrict to particular packages / modules, which looks like it will do the trick. Unfortunately my initial experiments with it haven't been fruitful - --include=PKG doesn't work whereas --include=PKG:MOD does. These issues don't cause errors or warning messages, though - instead you just get a coverage report of 0/0. I'll look into this more, later.

[mgsloan]

Got it, the package filter needs to look like --include=PKG-X-Y-Z:

[mgsloan]

@mboes How's this look?

[mboes]

LGTM.