Update the docs for the flutter_tools tests (#11218)

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 58fc585..eed673b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -134,7 +134,7 @@
 flutter shell. To run the test:
 
 * `cd packages/flutter_tools`
-* `dart --checked test/all.dart`
+* `pub run test -j1`
 
 The pre-built flutter tool runs in release mode with the observatory off by default.
 To enable debugging mode and the observatory on the `flutter` tool, uncomment the