[testing] Add initial documentation for test/message.

R=adamk@chromium.org, mstarzinger@chromium.org

BUG=

Change-Id: Ibac495e93b523bd034cc9f2d9e3a43cf38c9ab14
Reviewed-on: https://chromium-review.googlesource.com/463368Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44280}

test/message/message.md

+# JavaScript tests with expected output
+
+Tests in test/message pass if the output matches the expected output. Message
+tests are particularly useful when checking for exact error messages.
+
+Tests and their expected output must have the same filename, with the `.js` and
+`.out` extension.
+
+```
+foo.js
+foo.out
+```
+
+**All tests must end with an exception**. The test runner does not
+handle output from multiple runs, e.g.,  `--stress-opt`. Without an exception,
+the output will be generated several times and the comparison will fail.
+
+You can use a regex in the expected output. Instead of the exact
+path. use
+ ```
+*%(basename)s:7: SyntaxError: Detected cycle while resolving name 'a'
+```
+Empty lines are ignored in the comparison, but whitespaces are not.
+
+Exact details of the test runner are in [testcfg.py](testcfg.py).