[null-safety] remove mockito from scrollbar test (#64493)
[null-safety] remove mockito from scrollbar test (#64493)

More work towards #62886
tvolkert/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter