PUBDEV-8768 - fix testUniqueRowsWithCounts

[valenad1]

Original test throws away the last row because of wrongly defined chunk layout. This commit fix it and enhance the test.

I didn't touch input, only add correct reference and enhance asserting.

[michalkurka]

@valenad1 this would mean that the issue really only affects small datasets and in reasonably large datasets the values are accurate for all practical purposes, correct?

[michalkurka]

or is this just about making the test fail and no fix yet?

[valenad1]

@michalkurka unfortunately not fix yet, it turns out that the problem was in TestFrameBuilder and assertion. I am continue with this issue.