DEV: Only load specific plugin bundles during qunit test (#33678)

Previously, running qunit tests for a plugin would load the JS of all
installed plugins. This can be problematic because, depending on the
plugins installed in the current environment, there can be unexpected
interactions between the plugins.

This commit updates our qunit system to check the
`tests.requiredPlugins` list from the theme/plugin `about.json` file,
and only loads the listed plugins. This means that the environment is
much more consistent across CI and different development environments.

Alongside that change, this commit also:

- Starts storing the original `about.json` for themes in the database

- Improves qunit error handling when the test environment fails to boot
(e.g. there's a bad import in a plugin)

- Restores plugin CSS in theme qunit tests

.github/workflows/tests.yml

@@ -279,7 +279,7 @@ jobs:
 
       - name: Plugin QUnit
         if: matrix.build_type == 'frontend' && matrix.target == 'plugins'
-        run: QUNIT_WRITE_EXECUTION_FILE=1 bin/rake plugin:qunit['*']
+        run: bin/rake plugin:qunit['*']
 
       - name: Theme QUnit
         if: matrix.build_type == 'frontend' && matrix.target == 'themes'