test: reuse ffi.suffix instead of reimplementing it

ffi-test-common.js duplicated the platform branch already exposed
as ffi.suffix, risking drift if lib/ffi.js changes. Reuse it, and
require it after skipIfFFIMissing() so a build without FFI skips
instead of throwing.

Also assert ffi.suffix resolves to the correct value per platform,
since test-ffi-module.js only checked that the key exists.

Signed-off-by: leah-1ee <dltjddms1028@gmail.com>

Author: leah-1ee

test/ffi/ffi-test-common.js

@@ -6,18 +6,15 @@ const path = require('node:path');
 
 common.skipIfFFIMissing();
 
+const { suffix } = require('node:ffi');
+
 const fixtureBuildDir = path.join(
   __dirname,
   'fixture_library',
   'build',
   common.buildType,
 );
-const libraryPath = path.join(
-  fixtureBuildDir,
-  process.platform === 'win32' ? 'ffi_test_library.dll' :
-    process.platform === 'darwin' ? 'ffi_test_library.dylib' :
-      'ffi_test_library.so',
-);
+const libraryPath = path.join(fixtureBuildDir, `ffi_test_library.${suffix}`);
 
 function ensureFixtureLibrary() {
   if (!fs.existsSync(libraryPath)) {

test/ffi/test-ffi-module.js

@@ -154,6 +154,14 @@ test('ffi exports expected API surface', () => {
   assert.strictEqual(typeof ffi.types, 'object');
 });
 
+test('ffi.suffix matches the current platform', () => {
+  const ffi = require('node:ffi');
+  const expected = process.platform === 'win32' ? 'dll' :
+    process.platform === 'darwin' ? 'dylib' : 'so';
+
+  assert.strictEqual(ffi.suffix, expected);
+});
+
 test('ffi.types exports canonical type constants', () => {
   const ffi = require('node:ffi');
   const expected = {