Start a small playbook on how to deal with failing webtests. (#2962)

This is a starting point on how to deal with failures in the web-test
checks. They are supposed to warn for legitimate issues but if they
fail, they can be overridden.

The web-tests will be separated into its own job to better differentiate
if issues in building the English translation or web-testing the English
translation are happening

tests/README.md

@@ -17,7 +17,32 @@ when `npm start` or `npm test` is executed.
 > **Tip:** Use `cargo xtask web-tests` to run the tests in this directory from
 > anywhere in the repository.
 
-For local testing and quick iterations it is possible to use `mdbook serve`
+For local testing and quick iterations it is possible to use `cargo xtask serve`
 which creates a small HTTP server on port 3000 by default. There is a special
 config that is invoked with `npm run test-mdbook` that uses
 `http://localhost:3000`
+
+## Deal with failing tests
+
+When you see tests failing they should already indicate what checks broke for
+specific pages.
+
+### Legitimate warnings
+
+You might e.g. need to reduce the length of an overlong page (or get an
+exemption) or updating some mdbook infrastructure had a breaking change that
+breaks functionality. These issues need to be fixed before this change is
+merged.
+
+### Broken test environment
+
+Sometimes tests can also fail in the CI environment with errors like this.
+
+```
+ERROR webdriver: WebDriverError: tab crashed
+```
+
+If you see messages like these, it may indicate an issue with the web-tests that
+is not caused by your changes. Please file a bug to report this. As a temporary
+workaround, if all other checks pass and you are confident your changes are
+correct, you may override the web-test requirement to merge your pull request.