Rollup merge of #69765 - RalfJung:miri-test, r=LukasKalbertodt
Rollup merge of #69765 - RalfJung:miri-test, r=LukasKalbertodt

reduce test size for Miri

The larger sizes take quite a while, and there is probably little point in repeating this quite so often.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust