Removing MetaStruct Check from TestDirectoryListingLeak (#6404)

## Summary of changes
Same as #6395 this test
is part of the same family and I just noticed it also sometimes flakes
without the expected snapshot.

Author: link04

tracer/test/Datadog.Trace.Security.IntegrationTests/IAST/AspNetCore5IastTests.cs

@@ -742,7 +742,7 @@ public async Task TestDirectoryListingLeak(string featureEnvVar)
         newFixture.SetOutput(Output);
 
         var datetimeOffset = DateTimeOffset.UtcNow; // Catch vulnerability at the startup of the app
-        await TryStartApp(newFixture);
+        await TryStartApp(newFixture, new MockTracerAgent.AgentConfiguration { SpanMetaStructs = false });
 
         var agent = newFixture.Agent;
         var spans = agent.WaitForSpans(1, minDateTime: datetimeOffset);

tracer/test/snapshots/Iast.DirectoryListingLeak.AspNetCore5.IastEnabled.verified.txt

@@ -33,9 +33,6 @@
       _dd.top_level: 1.0,
       _dd.tracer_kr: 1.0,
       _sampling_priority_v1: 2.0
-    },
-    MetaStruct: {
-      iast: 
     }
   }
 ]