better way to ignore tests in Miri

src/offset_of.rs

@@ -138,7 +138,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(not(miri))] // this creates unaligned references
+    #[cfg_attr(miri, ignore)] // this creates unaligned references
     fn offset_simple_packed() {
         #[repr(C, packed)]
         struct Foo {

src/span_of.rs

@@ -177,7 +177,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(not(miri))] // this creates unaligned references
+    #[cfg_attr(miri, ignore)] // this creates unaligned references
     fn span_simple_packed() {
         #[repr(C, packed)]
         struct Foo {