update test readme

test/README.md

@@ -7,3 +7,18 @@ This folder contains the various test runners for VSCode. Please refer to the do
 * `integration`: our suite of API tests
 * `smoke`: our suite of automated UI tests
 * `ui`: our suite of manual UI tests
+
+
+### Browser Unit Tests
+
+```
+yarn test-browser --browser webkit --browser chromium
+```
+
+Unit tests from layers `common` and `browser` are run inside `chromium`, `webkit`, and (soon’ish) `firefox` (using playwright). This complements our electron-based unit test runner and adds more coverage of supported platforms. Notes:
+
+- these tests are part of the continuous build, that means you might have test failures that only happen with webkit on _windows_ or _chromium_ on linux
+- you can these tests locally via yarn `test-browser --browser chromium --browser webkit`
+- to debug, open  `<vscode>/test/unit/browser/renderer.html` inside a browser and use the `?m=<amd_module>`-query to specify what AMD module to load, e.g `file:///Users/jrieken/Code/vscode/test/unit/browser/renderer.html?m=vs/base/test/common/strings.test` runs all tests from `strings.test.ts`
+- to run only a subset of tests use the `--run` or `--glob` options
+