Add a release note snippet for the improved test setup selection.

docs/markdown/snippets/test-setups.md

@@ -0,0 +1,16 @@
+## Improve test setup selection
+
+Test setups are now identified (also) by the project they belong to and it
+is possible to select the used test setup from a specific project. E.g.
+to use a test setup `some_setup` from project `some_project` for all
+executed tests one can use
+
+    meson test --setup some_project:some_setup
+
+Should one rather want test setups to be used from the same project as
+where the current test itself has been defined, one can use just
+
+    meson test --setup some_setup
+
+In the latter case every (sub)project must have a test setup `some_setup`
+defined in it.