python/core-workflow#2: Add test coverage reporting for C files

[ammaraskar]

Nitty gritty on the details here: python/core-workflow#2 (comment)

[ammaraskar]

Relates to build process, can be marked with skip news and skip issue

[methane]

I afraid that test will go slower than Travis's time limit.
But we can split C coverage test and Python coverage test when it happened.

[ammaraskar]

Alternatively, we can also split the test suite in half and run them in seperate builds. Codecov takes care of merging the coverage results, so we could have the first ~200 or so tests run in one build and the remaining ~200 in another. I'll monitor the travis builds after this to make sure we don't hit the 50 minute limit.

[brettcannon]

It looks like it took 41 minutes, which still gives us a nearly 25% margin of error to complete within 50 minutes (although it did double the length of time it took to complete the coverage run).

I'm going to merge this and we can keep an eye on it going forward in case it ends up being an issue.

@ammaraskar thanks for doing this!