Add integration tests for vello_cpu

[LaurenzV]

Based on #830, which should be merged first.

I tried to follow the steps for Git LFS, but I'm not sure whether it actually worked.

[DJMcNab]

I think you'll probably need linebender/xilem@d4dca3e, although for slightly different reasons

[LaurenzV]

I mean, the other temporary option is to just store all screenshots in the same folder for now. Though I'm not sure how great it is to mix sparse_strip stuff with other vello stuff.

[xStrom]

Having a single vello_tests/lfs directory that contains all the LFS stuff does indeed seem like the simplest solution so we can just move on.

# Having all the LFS stuff with a single root path:
vello_tests/lfs
vello_tests/lfs/snapshots
vello_tests/lfs/refs

# Would mean we can just use:
vello_tests/lfs/**/*.png

I would go with that.

[LaurenzV]

Okay, I will try to do that tomorrow!