bindtests: Add a test for RBufferOut

Fixup the existing test, and add an entry in actions so it's actually generated.

generator/generator_actions.ml

@@ -55,6 +55,7 @@ let test_all_rets = [
   "test0rstruct",      RStruct ("valout", "lvm_pv");
   "test0rstructlist",  RStructList ("valout", "lvm_pv");
   "test0rhashtable",   RHashtable "valout";
+  "test0rbufferout",   RBufferOut "valout";
 ]
 
 let test_functions = [

generator/generator_bindtests.ml

@@ -181,6 +181,7 @@ print_strings (char *const *argv)
              pr "  strs[n*2] = NULL;\n";
              pr "  return strs;\n"
          | RBufferOut _ ->
+             pr "  *size_r = strlen (val);\n";
              pr "  return strdup (val);\n"
         );
         pr "}\n";