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

test/cases/return/return_int.tst

@@ -0,0 +1,4 @@
+return 42
+-- -- --
+s(:list,  [s(:return, 
+  s(:int,  42))])

test/cases/return/return_string.tst

@@ -0,0 +1,4 @@
+return "hello"
+-- -- --
+s(:list,  [s(:return, 
+  s(:string,  "hello"))])

test/cases/return/return_variable.tst

@@ -0,0 +1,4 @@
+return foo
+-- -- --
+s(:list,  [s(:return, 
+  s(:name,  "foo"))])