Rework the sample testing

[illwieckz]

Rework the sample testing.

That's something I wanted to do and that was prompted by:

• Ignore fully opaque alpha channels, preserve alpha when writing grayscale images #98
• Accept grayscale JPEGs with non-1x1 sampling factors #99

They even started to implement bits of that.

More changes than what was done in #98 and #98 were already wanted, but at the time I left them “for later”, “once those PR would be merged”. But in fact it's not a good strategy.

Now that I also wrote this:

• Endianness fixes #102

I want to add more tests and I need those helpers being there.

Doing the tests in #102 without this code will just mean to write code to entirely rewrite it once this is merged.

So the least costly way of doing it is to merge this first, then to rebase and merge those others (in that order):

• Ignore fully opaque alpha channels, preserve alpha when writing grayscale images #98
• Accept grayscale JPEGs with non-1x1 sampling factors #99
• Endianness fixes #102

[illwieckz]

I imported this commit:

• test: call wineserver -k before every wine run to avoid random errors

From:

• Ignore fully opaque alpha channels, preserve alpha when writing grayscale images #98

As it's generic test rewrite, so it better belongs here.