Add test for VK_EXT_memory_budget

Not ready yet (not even tested), but pushed so @jekstrand can see it :)

Tested and working now.

Outputs something like this:

[heap 0] size = 10793 MB, usage =   0 MB, budget = 10793 MB (at the start)
[heap 0] size = 10793 MB, usage =  64 MB, budget = 10793 MB (after allocating 64MB)
[heap 1] size =  1024 MB, usage =   0 MB, budget =  1024 MB (at the start)
[heap 1] size =  1024 MB, usage =  64 MB, budget =  1024 MB (after allocating 64MB)

And passes if (for each heap) the usage is at least 64MB higher after allocating 64MB.

(64MB was picked completely arbitrarily, happy to change it)