Rollup merge of #79631 - RalfJung:miri-const_str_ptr, r=oli-obk

disable a ptr equality test on Miri

This test relies on deduplication of constants. I do not think that this is a *guarantee* that Rust currently makes, and indeed Miri does not deduplicate constants the same way that rustc does, leading to different behavior in this test.

For now, I propose we simply disable this test in Miri.