Optimize expensive Miri tests

[joshlf]

Some tests are either marked #[cfg_attr(miri, ignore)] or have parameters set to smaller values under cfg!(miri). Even still, they are often expensive when executed, and it would be good to optimize them to save time in CI.

EDIT: It looks like this and related Miri optimizations are backwards, doing the expensive thing under Miri rather than the cheap thing:

zerocopy/src/lib.rs

Lines 3389 to 3393 in 524b2e2

	let assert_msg = if cfg!(miri) {
	format!("\n{args:?}\nsize:{size}, align:{align}")
	} else {
	String::new()
	};