chore: add actual vs. expected in error logging message

Closes #56988

GitOrigin-RevId: da1fceb
Change-Id: I1bed7fcdfdbf891f3205fb7ffd3e104fa6528cc8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392406
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: a-wallen

pkg/analysis_server/lib/src/g3/fixes.dart

@@ -192,7 +192,17 @@ class LintFixTesterWithSingleFix {
       fileEdit.edits,
     );
     if (actualFixedContent != fixedContent) {
-      throw StateError('Not expected content:\n$actualFixedContent');
+      throw StateError('''
+Expected the following content:
+```
+$fixedContent
+```
+
+but this was applied instead.
+```
+$actualFixedContent
+```
+''');
     }
   }