ci: Add an HTML report generator

ci: Add an HTML report generator

The JUnit cover report is useful, but only up to a point; for instance,
it's not used unless it's part of a merge request. This means you don't
get a report if you're pushing to a branch that does not have an MR open.

With a simple Python script and some minimal templating, we can generate
an HTML report from the "I Can't Believe it's not JSON™" log that Meson
produces, and keep it as a CI artifact.