test/ruby/test_string.rb: ensure we do not have tainted strings

Trying to figure out [ruby-core:82092]
cf. https://gist.github.com/e2c899c453f3f66e7934095c32505486

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59356 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_string.rb

@@ -2750,6 +2750,8 @@ CODE
     assert_not_equal(str.object_id, (+str).object_id)
     assert_equal(str.object_id, (-str).object_id)
     bar = %w(b a r).join('')
+    assert_not_predicate bar, :tainted?
+    assert_not_predicate str, :tainted?
     assert_same(str, -bar, "uminus deduplicates [Feature #13077]")
   end