docs: add code coverage documentation to test README

[hai1337]

Having a section on code coverage in test/README.md may be beneficial to guide contributors when writing code and tests.

Some details can include:

• Code coverage goal, e.g. >80% coverage (maintainers please define)
• Instructions to run coverage locally, e.g. pytest --cov=cve_bin_tool --cov-report html test/{TEST_FILE}.py
• Any other details such as functional coverage, branch coverage, exclusions, etc.

[terriko]

Some quick answers for anyone working on this:

• My current goal is ~80% coverage as reported by our existing tools
• We're currently using codecov.io so reports can be found be here: https://app.codecov.io/gh/intel/cve-bin-tool
  • It used to give us a heads up in a comment if coverage numbers dropped too much, but that apparently isn't working at the moment.
• I'm open to other tools in general, but because of the demand that GSoC puts on my time and the sheer number of open PRs that need shepherding, it's unlikely that I'll be making big tooling changes until late May or whenever we get down to < 5 PRs open. So if you want a pull request merged now, start with docs and propose tooling changes separately please!