BE: Tweak elp_development to prefer expect! in tests

Summary: As title

Reviewed By: robertoaloi

Differential Revision: D90984550

fbshipit-source-id: c4cbfb5887d8f11455d7afe683b0a03622425ec0

Author: alanz

.llms/rules/elp_development.md

@@ -134,7 +134,7 @@ when and how the diagnostic runs:
 
 ### Test Structure
 
-- Use `expect_test` for snapshot testing of complex outputs
+- Use `expect_test` and the `expect!` macro if possible
 - Group related tests in the same module
 - Use descriptive test names that explain the scenario