[Impeller] Sort AiksTests into category namespaces

[gaaclarke]

issue flutter/flutter#140106

This pulls out the following groups:

• gradients
• images
• pictures
• text
• clips
• paths
• unsorted

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[gaaclarke]

@matanlurey please give this a look. It isn't perfect but I think this will put us in a better spot than where we are today. I'll pull out a couple more groups tomorrow. This thing will be a pain to land since this file changes a lot (all the more reason to split it up at some point).

[matanlurey]

Supportive as an intermediate step.

My only ask (I can take a closer look tomorrow, EOD for me) is to add a header to the file explaining what the convention is if it will stay in this state for some time. Thanks for giving this a go!

[gaaclarke]

closing in lieu of the approach in #49050

[gaaclarke]

I found a better way to do this