add a test

tests/compile-fail/alignment.rs

@@ -0,0 +1,11 @@
+fn main() {
+    // miri always gives allocations the worst possible alignment, so a `u8` array is guaranteed
+    // to be at the virtual location 1 (so one byte offset from the ultimate alignemnt location 0)
+    let mut x = [0u8; 20];
+    let x_ptr: *mut u8 = &mut x[0];
+    let y_ptr = x_ptr as *mut u64;
+    unsafe {
+        *y_ptr = 42; //~ ERROR tried to access memory with alignment 1, but alignment
+    }
+    panic!("unreachable in miri");
+}