Fix some lints in LSP tests

Change-Id: Iff7e69d5d452a7bd405b727ab4966e1d71a89bda
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103531
Auto-Submit: Danny Tuppeny <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/test/lsp/initialization_test.dart

@@ -170,6 +170,7 @@ class InitializationTest extends AbstractLspAnalysisServerTest {
       const Duration(seconds: 1),
       onTimeout: () {
         didTimeout = true;
+        return null;
       },
     );
 

pkg/analysis_server/test/lsp/server_test.dart

@@ -75,6 +75,7 @@ class ServerTest extends AbstractLspAnalysisServerTest {
       const Duration(seconds: 1),
       onTimeout: () {
         didTimeout = true;
+        return null;
       },
     );