Adapt rustc_data_structures tests to run in strict miri

Some tests took too long and owning_ref is fundamentally flawed,
so don't run these tests or run them with a shorter N. This makes
miri with `-Zmiri-strict-provenance` usable to find UB.

compiler/rustc_data_structures/src/base_n/tests.rs

@@ -15,7 +15,9 @@ fn test_encode() {
         test(u64::MAX as u128, base);
         test(u128::MAX, base);
 
-        for i in 0..1_000 {
+        const N: u128 = if cfg!(miri) { 10 } else { 1000 };
+
+        for i in 0..N {
             test(i * 983, base);
         }
     }

compiler/rustc_data_structures/src/graph/scc/tests.rs

@@ -156,7 +156,10 @@ fn test_deep_linear() {
     v
     …
      */
+    #[cfg(not(miri))]
     const NR_NODES: usize = 1 << 14;
+    #[cfg(miri)]
+    const NR_NODES: usize = 1 << 3;
     let mut nodes = vec![];
     for i in 1..NR_NODES {
         nodes.push((i - 1, i));

compiler/rustc_data_structures/src/owning_ref/tests.rs

@@ -1,3 +1,5 @@
+// FIXME: owning_ref is not sound under stacked borrows. Preferably, get rid of it.
+#[cfg(not(miri))]
 mod owning_ref {
     use super::super::OwningRef;
     use super::super::{BoxRef, Erased, ErasedBoxRef, RcRef};
@@ -361,6 +363,8 @@ mod owning_handle {
     }
 }
 
+// FIXME: owning_ref is not sound under stacked borrows. Preferably, get rid of it.
+#[cfg(not(miri))]
 mod owning_ref_mut {
     use super::super::BoxRef;
     use super::super::{BoxRefMut, Erased, ErasedBoxRefMut, OwningRefMut};