[PRISM] Enable TestSyntax#test_unterminated_heredoc_cr

test/.excludes-prism/TestSyntax.rb

@@ -1,5 +1,4 @@
 exclude(:test_dedented_heredoc_continued_line, "heredoc line continuation dedent calculation")
-exclude(:test_unterminated_heredoc_cr, "quoted \r heredoc terminators should not match \r\n")
 
 exclude(:test_duplicated_when, "https://bugs.ruby-lang.org/issues/20401")
 exclude(:test_optional_self_reference, "https://bugs.ruby-lang.org/issues/20478")