Exclude MRI test broken without RubyVM::InstructionSequence.compile

* This is what CRuby does, which would fail the test due to different message:
  ruby -e 'p self.then { eval "next #{rand}" }'
  (eval): (eval):1: Can't escape from eval with next (SyntaxError)

Author: eregon

test/mri/excludes/TestSyntax.rb

@@ -54,3 +54,4 @@
 exclude :test_methoddef_endless, "needs investigation"
 exclude :test_numbered_parameter, "needs investigation"
 exclude :test_warning_for_cr, "needs investigation"
+exclude :test_invalid_next, "#syntax_check does catch {|tag| eval(code, binding, ...) } and so next is arguably valid since it's in a block"