Disable a text field test that fails on some Macs with libtxt (#14895)

packages/flutter/test/material/text_field_test.dart

@@ -737,8 +737,8 @@ void main() {
     expect(inputBox.hitTest(new HitTestResult(), position: inputBox.globalToLocal(newFirstPos)), isTrue);
     expect(inputBox.hitTest(new HitTestResult(), position: inputBox.globalToLocal(newFourthPos)), isFalse);
   },
-  // This test does not run as expected in the MacOS environment on Travis
-  skip: runningOnTravis && Platform.isMacOS);
+  // This test fails on some Mac environments when libtxt is enabled.
+  skip: Platform.isMacOS);
 
   testWidgets('TextField smoke test', (WidgetTester tester) async {
     String textFieldValue;

packages/flutter/test/painting/text_painter_rtl_test.dart

@@ -320,8 +320,8 @@ void main() {
       ],
     );
   },
-  // Ahem-based tests don't yet quite work on Windows or the MacOS environment in Travis
-  skip: Platform.isWindows || (runningOnTravis && Platform.isMacOS));
+  // Ahem-based tests don't yet quite work on Windows or some MacOS environments
+  skip: Platform.isWindows || Platform.isMacOS);
 
   test('TextPainter - line wrap mid-word', () {
     final TextPainter painter = new TextPainter()

packages/flutter/test/rendering/paragraph_test.dart

@@ -70,8 +70,8 @@ void main() {
     expect(boxes.any((ui.TextBox box) => box.left == 250 && box.top == 0), isTrue);
     expect(boxes.any((ui.TextBox box) => box.right == 100 && box.top == 10), isTrue);
   },
-  // Ahem-based tests don't yet quite work on Windows or the MacOS environment in Travis
-  skip: Platform.isWindows || (runningOnTravis && Platform.isMacOS));
+  // Ahem-based tests don't yet quite work on Windows or some MacOS environments
+  skip: Platform.isWindows || Platform.isMacOS);
 
   test('getWordBoundary control test', () {
     final RenderParagraph paragraph = new RenderParagraph(