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>
PR-URL: #64840
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Aviv Keller <me@aviv.sh>

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');
+
 constfixtureBuildDir=path.join(
 __dirname,
 'fixture_library',
 'build',
 common.buildType,
 );
-constlibraryPath=path.join(
-fixtureBuildDir,
-process.platform==='win32' ? 'ffi_test_library.dll' :
-process.platform==='darwin' ? 'ffi_test_library.dylib' :
-'ffi_test_library.so',
-);
+constlibraryPath=path.join(fixtureBuildDir,`ffi_test_library.${suffix}`);
 
 functionensureFixtureLibrary(){
 if(!fs.existsSync(libraryPath)){

test/ffi/test-ffi-module.js

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