reverted tests to cope with "regression" back to printing leading core::

The reason we see `core::` even after visiting the `std` crate first
is the special case code that looks like this:

```rust
    Entry::Occupied(mut entry) => {
        // If `child` is defined in crate `cnum`, ensure
        // that it is mapped to a parent in `cnum`.
        if child.krate == cnum && entry.get().krate != cnum {
            entry.insert(parent);
        }
    }
```

This causes items to be associated with the crates they were
originally defined in, even if we had encountered them during the
traversal of an earlier crate.

(Having said that, I am not clear on why this same logic does not
apply when both rust-lang#46708 and rust-lang#46838 have been applied. But at this
point, I am just happy to have a plausible explanation for why we see
`core::foo::bar` in the output for these tests, and want to focus on
getting this fix for rust-lang#46112 backported to beta.)

Author: pnkfelix

src/test/compile-fail/issue-17959.rs

@@ -19,7 +19,7 @@ struct G<T: ?Sized> {
 }
 
 impl<T> Drop for G<T> {
-//~^ ERROR: The requirement `T: std::marker::Sized` is added only by the Drop impl. [E0367]
+//~^ ERROR: The requirement `T: core::marker::Sized` is added only by the Drop impl. [E0367]
     fn drop(&mut self) {
         if !self._ptr.is_null() {
         }

src/test/compile-fail/kindck-send-unsafe.rs

@@ -14,7 +14,7 @@ fn assert_send<T:Send>() { }
 
 fn test71<'a>() {
     assert_send::<*mut &'a isize>();
-    //~^ ERROR `*mut &'a isize: std::marker::Send` is not satisfied
+    //~^ ERROR `*mut &'a isize: core::marker::Send` is not satisfied
 }
 
 fn main() {

src/test/ui/print_type_sizes/niche-filling.stdout

@@ -69,11 +69,11 @@ print-type-size type: `MyOption<bool>`: 1 bytes, alignment: 1 bytes
 print-type-size     variant `None`: 0 bytes
 print-type-size     variant `Some`: 1 bytes
 print-type-size         field `.0`: 1 bytes
-print-type-size type: `MyOption<std::cmp::Ordering>`: 1 bytes, alignment: 1 bytes
+print-type-size type: `MyOption<core::cmp::Ordering>`: 1 bytes, alignment: 1 bytes
 print-type-size     variant `None`: 0 bytes
 print-type-size     variant `Some`: 1 bytes
 print-type-size         field `.0`: 1 bytes
-print-type-size type: `std::cmp::Ordering`: 1 bytes, alignment: 1 bytes
+print-type-size type: `core::cmp::Ordering`: 1 bytes, alignment: 1 bytes
 print-type-size     discriminant: 1 bytes
 print-type-size     variant `Less`: 0 bytes
 print-type-size     variant `Equal`: 0 bytes