Clean up test infrastructure (#43030)

See #41880 for history.

dev/README.md

diff --git a/dev/README.md b/dev/README.md
index afe8f26..3c287dd 100644
--- a/dev/README.md
+++ b/dev/README.md
@@ -2,3 +2,6 @@
 during development of the framework. The tools in this directory
 should not be necessary for developing Flutter applications, though of
 course they may be interesting if you are curious.
+
+The tests in this directory are run in the `framework_tests_misc-*`
+shards.