Please, CI needs to run all plugins tests OR remind to add testsuite when new plugin is added

In https://github.com/haskell/haskell-language-server/pull/2530 lead to: https://github.com/haskell/haskell-language-server/pull/2530#issuecomment-1000236484

On adding a new `plugin` - people on both sides of the review can forget (& CI would give a wrong impression that everything passes) that CI does not have plugin check. So CI actually does not check that the plugin works properly & contributor & reviewer can merge it.

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1. CI conf can be done in a way that it automatically runs/generates test suite runs `forall` plugins.
2. When the new directory into `plugins` is added, or when the list of dirs there do not match the literal list - remind to add the test suite hook into `test.

Review the CI configuration, the `plugins` directory layout, and the test-suite hooks referenced in the issue, starting with the context from PR 2530. Decide whether CI should run all plugin tests or detect missing hooks when plugin directories change. Done means CI no longer gives a misleading pass when a plugin lacks test coverage or a clear reminder is produced.

Written by the indexing model from the issue text.