test: update postmortem metadata test

This commit updates the following postmortem metadata constant:

- v8dbg_bit_field3_dictionary_map_shift
  - This is now v8dbg_bit_field3_is_dictionary_map_shift as of
    v8/v8@7a159da

Refs: #34

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-postmortem-metadata.js

@@ -44,7 +44,7 @@ function getExpectedSymbols() {
     // should only consist of postmortem constants, and some of them can be
     // relatively long.
     /* eslint-disable max-len */
-    'v8dbg_bit_field3_dictionary_map_shift',
+    'v8dbg_bit_field3_is_dictionary_map_shift',
     'v8dbg_bit_field3_number_of_own_descriptors_shift',
     'v8dbg_class_Code__instruction_size__int',
     'v8dbg_class_Code__instruction_start__uintptr_t',