test: update postmortem metadata test

This commit updates the following postmortem metadata constant:

- v8dbg_context_idx_closure
  - Renamed: v8dbg_context_idx_scope_info
  - V8 commit: v8/v8@39496a9#diff-f3f182b0510ba2ee39ae87e421ff110b

Fixes: nodejs/node-v8#59

Backport-PR-URL: #21668
PR-URL: #21079
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>

test/v8-updates/test-postmortem-metadata.js

@@ -105,7 +105,7 @@ function getExpectedSymbols() {
     'v8dbg_class_SlicedString__parent__String',
     'v8dbg_class_String__length__SMI',
     'v8dbg_class_ThinString__actual__String',
-    'v8dbg_context_idx_closure',
+    'v8dbg_context_idx_scope_info',
     'v8dbg_context_idx_prev',
     'v8dbg_context_min_slots',
     'v8dbg_frametype_ArgumentsAdaptorFrame',