test: optionally set common.PORT via env variable
diff --git a/test/common.js b/test/common.js
index 455a4cd..d529ab2 100644
--- a/test/common.js
+++ b/test/common.js
@@ -26,7 +26,7 @@
 exports.fixturesDir = path.join(exports.testDir, 'fixtures');
 exports.libDir = path.join(exports.testDir, '../lib');
 exports.tmpDir = path.join(exports.testDir, 'tmp');
-exports.PORT = 12346;
+exports.PORT = +process.env.NODE_COMMON_PORT || 12346;
 
 if (process.platform === 'win32') {
   exports.PIPE = '\\\\.\\pipe\\libuv-test';