ci: Move db820c and db410c's gles3 tests to manual, like radv did.

Merged Emma Anholt requested to merge anholt/mesa:ci-db820c-no-gles3 into master

This should make these tests available for clicking on the web ui in personal branches, while hiding them from marge and the post-merge CI pipelines. We had already disabled db410c's gles3, but it wasn't available in the ui and you had to hack .gitalb-ci.yml. db820c is now being disabled by default, due to instaboots mentioned in #2649 (closed)

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