test: skip sub-page granularity smoke test under miri

Mirrors the cfg(not(miri)) attribute already applied to the equivalent
internal test in src/dlmalloc.rs. With 32-byte granularity, chunks pack
tightly enough to hit small-bin unlink paths that trip dlmalloc-rs's
pre-existing Stacked Borrows quirk with self-aliasing in smallbins.

Author: Arthur Komatsu

tests/smoke.rs

@@ -76,7 +76,13 @@ fn configurable_api_smoke() {
 // so this primarily exercises the dlmalloc-side accounting; on the
 // embedded targets this PR is motivated by, it also packs allocations
 // tightly into the application heap.
+// Skipped under miri: with 32-byte granularity, chunks are packed tightly
+// enough that small-bin unlink paths get exercised in a way that trips
+// dlmalloc-rs's pre-existing Stacked Borrows quirk with `smallbins`
+// self-aliasing (the internal `custom_sub_page_granularity_alloc_free`
+// test in `src/dlmalloc.rs` is skipped for the same reason).
 #[test]
+#[cfg(not(miri))]
 fn sub_page_granularity_alloc_free() {
     let sub_page = 4 * core::mem::size_of::<usize>();
     let mut a = Dlmalloc::new_with_config(sub_page, 4095);