Remove duplicated build infrastructure tests

[handrews]

This is a bit of missed clean-up from the build-infra migration. Note that we are keeping scripts in tests/schema because those are custom per repository. The md2html tests are shared and already supported by build-infra.

Keep renderer implementation tests in build-infra and make the schema workflow run only the repository-owned schema suite when it is present.

Commit ee45561c:

• Removes the entire duplicated tests/md2html tree.
• Makes schema CI run tests/schema/schema.test.mjs explicitly when present.
• Leaves schema CI successful on main, where that runner is intentionally absent.
• Updates CONTRIBUTING.md to describe yarn test as the development-branch schema suite.

Verification:

• Markdown and link validation passed.

• The explicit command ran all 20 dev schema tests successfully.

• Workflow YAML and Git diffs passed validation.

• ReSpec’s local browser launch failed due to the execution environment; it reached the build step and is unrelated to this cleanup.

• Working tree is clean. Nothing was pushed.

• schema changes are included in this pull request

• schema changes are needed for this pull request but not done yet

• no schema changes are needed for this pull request

[handrews]

Something else is needed here to get the tests from the build-infra package to run.

And of course linkspector is confused again, even though every single link it complains about is fine. Why do we use this thing? It's been nothing but pain.

[handrews]

A new version of this will get posted after OAI/build-infra#65 is merged and released.