Update test instructions for flutter_tools (#8126)

flutter_tools testing requires the FLUTTER_TOOLS environment variable to
be set.

* Add note about ensuring that no devices are connected

packages/flutter_tools/README.md

@@ -2,7 +2,9 @@
 
 Tools for building Flutter applications.
 
-To run the tests:
+To run the tests, ensure that no devices are connected and run:
 
-- pub get
-- pub run test
+```shell
+pub get
+FLUTTER_ROOT=$PWD/../.. dart --checked test/all.dart
+```