test: keep finalization close fixture ref alive

The close fixture expects the registered object to still be reachable
when the process emits exit. Keep a strong reference outside setup() so
the assertion does not depend on platform-specific GC timing.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #64085
Refs: https://github.com/nodejs/reliability/issues?q=%22test-process-finalization%22
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Stefan Stojanovic <stefan.stojanovic@janeasystems.com>

Author: trivikr

test/fixtures/process/close.mjs

@@ -1,7 +1,9 @@
 import { strictEqual } from 'assert'
 
+let obj
+
 function setup() {
-  const obj = { foo: 'bar' }
+  obj = { foo: 'bar' }
   process.finalization.register(obj, shutdown)
 }
 
@@ -14,5 +16,6 @@ function shutdown(obj) {
 setup()
 
 process.on('exit', function () {
+  strictEqual(obj.foo, 'bar')
   strictEqual(shutdownCalled, true)
 })