[flutter_tools] tweak README (#84648)

packages/flutter_tools/README.md

@@ -42,7 +42,8 @@ Tests live under the `test/` subdirectory.
 - Hermetic unit tests of tool internals go under `test/general.shard`.
 - Tests of tool commands go under `test/commands.shard`. Hermetic tests go under
   its `hermetic/` subdirectory. Non-hermetic tests go under its `permeable`
-  sub-directory.
+  sub-directory. Avoid adding tests here and prefer writing either a unit test or a full
+  integration test.
 - Integration tests (e.g. tests that run the tool in a subprocess) go under
   `test/integration.shard`.
 
@@ -66,18 +67,20 @@ flutter test test/integration.shard/some_test_case
 
 ### Running the tests
 
-To run the tests in the `test/` directory:
+To run all of the unit tests:
 
 ```shell
-$ flutter test
+$ flutter test test/general.shard
 ```
 
 The tests in `test/integration.shard` are slower to run than the tests in
 `test/general.shard`. Depending on your development computer, you might
-want to increase timeouts and limit concurrency.
+want to increase timeouts and limit concurrency. Generally it is easier to run
+these on CI, or to manually verify the behavior you are changing instead of running
+the test.
+
 The integration tests also require the `FLUTTER_ROOT` environment variable
-to be set.
-The full invocation to run everything might therefore look something like:
+to be set. The full invocation to run everything might therefore look something like:
 
 ```shell
 $ FLUTTER_ROOT=~/path/to/flutter-sdk