change test framework to use files for in and out (s-exp)

test/cases/function_definition/class_function.tst

@@ -0,0 +1,5 @@
+int  String.length( ref x )
+  length
+end
+-- -- --
+{:expression_list=>[{:type=>"int", :receiver=>s(:module,  "String"), :function_name=>s(:name,  "length"), :parameter_list=>[s(:field,  :ref,  :x)], :expressions=>[s(:name,  "length")], :end=>"end"}]}