message: support fetch test case in parser
diff: @@ -57,5 +57,5 @@ nebula> GO FROM 1 OVER e1 YIELD e1.prop1 -- for all the out going edges of
  nebula> GO FROM 1 OVER e1 YIELD e1._src AS s, serve._dst AS d \
  | FETCH PROP ON e1 $-.s -> $-.d YIELD e1.prop1 -- the same as above sentence
  nebula> $var = GO FROM 1 OVER e1 YIELD e1._src AS s, e2._dst AS d;\
  - FETCH PROP ON serve $var.s -> $var.d YIELD e1.prop1. -- the same as above.
  + FETCH PROP ON e3 $var.s -> $var.d YIELD e3.prop1. -- the same as above.
  ```
  