[Mac] Fixed font unit tests
Misc unit test fixes

* FontTests.cs: Added test for font weight rendering

* LabelTests.cs: Minor unit test issue

* Label.AlignCenterWrappedChangeText.png: Update test images

* TransformTests.cs: Use a bigger delta when comparing results,
  required for Mac.
[Mac] Label must have an empty string by default
[Mac] Fix crash in ComboBoxEntry