Generate C code coverage reports

[ganesh-k13]

Proposal

Add two new options to spin test:

• --generate-lcov-html

• --generate-lcov-xml

          I just realised `--generate-lcov-html` can also be part of `spin` itself instead of the individual downstreams implementing it as it's controlled by `ninja` and it's not a custom implementation using `lcov` as it was before in NumPy. Let me PR that when I'm back.
  

Originally posted by @ganesh-k13 in #146 (comment)