runner: support "includes" from YAML frontmatter

support frontmatter "includes" in python runner
use test.includes if present instead of scanning test code with regex

This PR adds **theoretical** support for the `includes` YAML.  No tests currently use it, so it is untested.

I will add some test fixtures later and inspect the packaging code to make sure the `includes` is passed from the test frontmatter into the generated JSON, where the browser runner can read it.

In both the python and in-browser runners, this patch falls back to scanning for `$INCLUDE("")` directives if the `test.includes` member is absent.