add exposed_only_ro test

tests/fail/stacked_borrows/exposed_only_ro.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Zmiri-permissive-provenance
+#![feature(strict_provenance)]
+
+// If we have only exposed read-only pointers, doing a write through a wildcard ptr should fail.
+
+fn main() {
+    let mut x = 0;
+    let _fool = &mut x as *mut i32; // this would have fooled the old untagged pointer logic
+    let addr = (&x as *const i32).expose_addr();
+    let ptr = std::ptr::from_exposed_addr_mut::<i32>(addr);
+    unsafe { ptr.write(0) }; //~ ERROR: borrow stack
+}