Prevent "warning: ambiguity between regexp and two divisions"

http://rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20230221T031004Z.log.html.gz
```
/home/chkbuild/chkbuild/tmp/build/20230221T031004Z/ruby/test/rubygems/test_gem_ext_cargo_builder.rb:90: warning: ambiguity between regexp and two divisions: wrap regexp in parentheses or add a space after `/' operator
```

test/rubygems/test_gem_ext_cargo_builder.rb

@@ -87,7 +87,7 @@ class TestGemExtCargoBuilder < Gem::TestCase
       end
     end
 
-    assert_match /cargo\s.*\sfailed/, error.message
+    assert_match(/cargo\s.*\sfailed/, error.message)
   end
 
   def test_full_integration