Separate parsing of test results from report generation

[chrisdarroch]

This refactor makes it simpler to consume this repository as a set of tests and a test runner for the various JS environments. The build.js module remains responsible for constructing the compatibility table, where the test-parser.js module is responsible for prepping individual assertions for checking in a given environment.

I've done what I can to preserve the git history after splitting the files so that git blame continues to work in both files.

[chrisdarroch]

Is there anything further I can do to get this PR merged?