Enhance testing conventions with specific examples and diagnostics guidance

Added sub-bullets to testing conventions explaining:
- Use concise methods like Single() instead of asserting count and extracting elements
- Always validate diagnostics in compiler tests for reviewer clarity

Co-authored-by: CyrusNajmabadi <4564579+CyrusNajmabadi@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -56,6 +56,8 @@ var symbolInfo = semanticModel.GetSymbolInfo(expression);
 - Add `[WorkItem("https://github.com/dotnet/roslyn/issues/issueNumber")]` attribute to tests that fix specific GitHub issues
 - Prefer raw string literals (`"""..."""`) over verbatim strings (`@"..."`) when creating test source code
 - Avoid unnecessary intermediary assertions - tests should do the minimal amount of work to validate just the core issue being addressed
+  - Use concise methods like `.Single()` instead of asserting count and extracting elements
+  - For compiler tests, validate diagnostics (e.g., `comp.VerifyDiagnostics()`) so reviewers can easily see if the code is in error or represents something legal
 
 ## Critical Integration Points