Please review a testbug fix for an issue whereby it seen that it fails
citing "failing to hide opaque button".
It is seen that the test compares the buffered image strictly
pixel-by-pixel and even if it differs by 1,2 pixels it fails which is
what was happening here.
Proposed fix is to make it more lenient by allowing a tolerance of
20pixels difference in the comparison of 2 bufferedimage.
It is verified on windows, mac.