[PRISM] Enable TestSyntax#test_warn_balanced

test/.excludes-prism/TestSyntax.rb

@@ -1,7 +1,6 @@
 exclude(:test_dedented_heredoc_continued_line, "heredoc line continuation dedent calculation")
 exclude(:test_it, "https://github.com/ruby/prism/issues/2323")
 exclude(:test_unterminated_heredoc_cr, "quoted \r heredoc terminators should not match \r\n")
-exclude(:test_warn_balanced, "missing warning for ** being interpreted as a binary operator")
 
 exclude(:test_duplicated_when, "https://bugs.ruby-lang.org/issues/20401")
 exclude(:test_optional_self_reference, "https://bugs.ruby-lang.org/issues/20478")