Add additional expects to check if LineMetrics flake is a race (#49261)

packages/flutter/test/painting/text_painter_test.dart

diff --git a/packages/flutter/test/painting/text_painter_test.dart b/packages/flutter/test/painting/text_painter_test.dart
index e2996f0..670fdc5 100644
--- a/packages/flutter/test/painting/text_painter_test.dart
+++ b/packages/flutter/test/painting/text_painter_test.dart
@@ -741,6 +741,9 @@
 
     painter.layout(maxWidth: 300);
 
+    expect(painter.text, const TextSpan(text: text));
+    expect(painter.preferredLineHeight, 14);
+
     final List<ui.LineMetrics> lines = painter.computeLineMetrics();
 
     expect(lines.length, 4);