Fix tests for changes in #26241.

Author: SimonSapin

src/libcollections/str.rs

@@ -780,7 +780,7 @@ impl str {
     ///
     /// # Examples
     /// ```
-    /// # #![feature(collections)]
+    /// # #![feature(str_split_at)]
     /// let s = "Löwe 老虎 Léopard";
     /// let first_space = s.find(' ').unwrap_or(s.len());
     /// let (a, b) = s.split_at(first_space);

src/libcollectionstest/lib.rs

@@ -44,6 +44,7 @@
 #![feature(str_char)]
 #![feature(str_escape)]
 #![feature(str_match_indices)]
+#![feature(str_split_at)]
 #![feature(str_utf16)]
 #![feature(box_str)]
 #![feature(subslice_offset)]

src/test/compile-fail/str-mut-idx-2.rs

@@ -13,10 +13,11 @@ fn bot<T>() -> T { loop {} }
 fn mutate(s: &mut str) {
     s[1..2] = bot();
     //~^ ERROR `core::marker::Sized` is not implemented for the type `str`
-    //~^^ ERROR `core::marker::Sized` is not implemented for the type `str`
+    //~| ERROR `core::marker::Sized` is not implemented for the type `str`
     s[1usize] = bot();
     //~^ ERROR `core::ops::Index<usize>` is not implemented for the type `str`
-    //~^^ ERROR `core::ops::Index<usize>` is not implemented for the type `str`
+    //~| ERROR `core::ops::IndexMut<usize>` is not implemented for the type `str`
+    //~| ERROR `core::ops::Index<usize>` is not implemented for the type `str`
 }
 
 pub fn main() {}