test: wait for FFI background optimization

The pointer range test creates multiple closures from the same function
literals and explicitly requests synchronous optimization. V8 can also
schedule concurrent recompilation for those closures.

Wait for background optimization before closing the dynamic library so
compiler jobs cannot outlive the fast FFI metadata they reference.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: codex:gpt-5.6-sol
PR-URL: #65300
Refs: https://github.com/nodejs/reliability/issues?q=sort%3Aupdated-desc%20%22test-ffi-fast-integer-validation%22
Reviewed-By: Aviv Keller <me@aviv.sh>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>

Author: trivikr

test/ffi/test-ffi-fast-integer-validation.js

@@ -65,6 +65,7 @@ test('fast FFI validates integer argument ranges', () => {
     assert.throws(() => callI64(2n ** 63n), expect);
     assert.throws(() => callU64(2n ** 64n), expect);
   } finally {
+    eval('%WaitForBackgroundOptimization()');
     lib.close();
   }
 });
@@ -96,6 +97,7 @@ test('fast FFI validates pointer BigInt ranges', () => {
       }
     }
   } finally {
+    eval('%WaitForBackgroundOptimization()');
     lib.close();
   }
 });