Adjust integration test for upstream changes

See rust-lang/rust#6745.

Author: shepmaster

tests/spec/features/tools_spec.rb

@@ -61,7 +61,7 @@ def code_with_undefined_behavior
 
     within(:output, :stdout) do
       # First-party
-      expect(page).to have_content('core::fmt::Arguments::new_v1')
+      expect(page).to have_content('::std::io::_print')
 
       # Third-party procedural macro
       expect(page).to have_content('block_on(body)')