Add Tests to verify the count of colors returned matches the count requested

[ijambro]

I wanted to let you know that for values of count greater than 7, the actual number of colors returned is one fewer than the requested count.

I added tests here to automate checking for the issue. The first two tests pass, and the final test fails: Uncaught AssertionError [ERR_ASSERTION]: 9 == 10

I've debugged the cause: The count that is received by quantize is considered a maximum count, but in get-image-colors and get-rgba-palette, we treat it as THE count.

I think quantize should be able to better guarantee the actual colors returned matches the count requested, so I opened an Issue and a Pull Request against them: https://github.com/olivierlesnicki/quantize/pull/10