Update tests and generate coverage

[mdpiper]

The goal of this PR is to update the unit tests for the babelizer and generate code coverage statistics.

One big, questionable, decision: I moved the language-specific tests under the external directory. My rationale is that these tests require the language examples to be built, which takes several extra steps. Only language-agnostic tests remain under the top-level tests directory. I made rules to handle both in the Makefile.

Summary of changes:

• Moved language tests under the external directory
• Made new language-agnostic tests in the tests directory
• Made a babelize update test with the Python example
• Generated coverage for all tests in the GitHub Actions CI

This fixes #53.

[mdpiper]

@mcflugen Please let me know what you think!