Testmatrix: Add a build where C extensions are compiled without optimizations

[MSeifert04]

The issue in #8986 is probably something that we should've catched with out CI. The problem there was that the implementation assumed that the function is always going to be inlined, however when there is a debug build or when the compiler decides not to inline it then the code is going to fail.

I assumed that we used debug builds for the C coverage report but given that these didn't fail we probably have at least some optimizations "on" there.

[mhvk]

Good idea to have a debug build - in numpy those often found bugs. Presumably we can convert one of the regular builds (coverage sounds good)

[bsipocz]

@astrofrog @pllim - this is one of those package-novice, good first issues that are also effort low for someone familiar with CI and compilers, but one that I am not comfortable to label as package-novice, so let's try to figure out a new label.

[pllim]

Re: new label -- @bsipocz , I thought we used to have a "domain expert" label but I don't see it now.

[MSeifert04]

Currently our coverage build does use O0 and -fn-inline-functions so maybe the issue is in the compiler (or only observable in some compilers): https://github.com/astropy/astropy/blob/master/.travis.yml#L127-L134

[astrofrog]

Domain expert isn't the right one though, that was more for astronomy expertise. Maybe having something like infrastructure-expert or something would be good here. I think package-novice is the right label to have, it's just we need additional labels to indicate we need good python or other expertise.

[bsipocz]

technical-expert? not really the right one either.