docs: tips for tests [skip ci]

Author: wkillerud

docs/contributing/adding-new-features.md

@@ -18,5 +18,7 @@ A summary of what you need:
    - Look at existing features for some common patterns, such as how to parse a `TextDocument` to get the AST.
    - You may want to know what AST node is at a given `Position`. See `node_at_offset_visitor.dart`.
    - Use `findInWorkspace` to run a callback on each linked document, recursively (with a `lazy` option).
+   - Write [tests](../../pkgs/sass_language_services/test/features/) using the [memory file system](../../pkgs/sass_language_services/test/memory_file_system.dart).
 7. Add the feature to the public API in [language_services.dart](../../pkgs/sass_language_services/lib/src/language_services.dart).
 8. Use the feature in the request handler in `language_server.dart`.
+9. Add a VS Code test as an an end-to-end "smoketest" in the [extension](../../extension/test/README.md).

extension/test/README.md

@@ -8,8 +8,8 @@ Each subdirectory in `electron/` will run in a separate instance of VS Code.
 
 By convention subdirectories in `electron/` should have:
 
-- `index.js` as the entrypoint that finds test files and passes them to Mocha
-- at least one `*.test.js` file with some tests
-- a `fixtures/` subdirectory with:
-  - `.vscode/settings.json`
-  - `styles.scss` or `styles.sass`
+- `index.js` as the entrypoint that finds test files and passes them to Mocha.
+- At least one `*.test.js` file with some tests.
+- A `fixtures/` subdirectory with:
+  - `.vscode/settings.json` with any non-default settings you want to test.
+  - `styles.scss` or `styles.sass`.