Re-enable CI on stable channel (#3774)

Author: srawlins

.github/workflows/test.yaml

@@ -22,9 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        # TODO(srawlins): Re-enable stable after 3.4.0 ships.
-        # sdk: [dev, stable]
-        sdk: [dev]
+        sdk: [dev, stable]
         job: [main, flutter, packages, sdk-docs]
         include:
           - os: macos-latest