Attempt to fix flaky filestream parser tests. (#31773)

Poll on the registry having the expected state instead of just looking
once after the expected number of events were propagated through the
pipeline. Fixes the flakiness that could be reproduced locally.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit b502a78)

filebeat/input/filestream/environment_test.go

@@ -203,11 +203,16 @@ func (e *inputTestingEnvironment) requireOffsetInRegistry(filename, inputID stri
 	}
 
 	id := getIDFromPath(filepath, inputID, fi)
-	entry, err := e.getRegistryState(id)
-	if err != nil {
-		e.t.Fatalf(err.Error())
-	}
+	var entry registryEntry
+	require.Eventually(e.t, func() bool {
+		entry, err = e.getRegistryState(id)
+		if err != nil {
+			return true
+		}
 
+		return expectedOffset == entry.Cursor.Offset
+	}, time.Second, time.Millisecond)
+	require.NoError(e.t, err)
 	require.Equal(e.t, expectedOffset, entry.Cursor.Offset)
 }