test: allow common.PIPE to be configured via env

Add optional env var $NODE_COMMON_PIPE for setting common.PIPE to
manually deal with maximum path lengths for unix sockets.

PR-URL: https://github.com/node-forward/node/pull/26
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/common.js

@@ -37,6 +37,8 @@ if (process.platform === 'win32') {
 } else {
   exports.PIPE = exports.tmpDir + '/test.sock';
 }
+if (process.env.NODE_COMMON_PIPE)
+  exports.PIPE = process.env.NODE_COMMON_PIPE;
 if (!fs.existsSync(exports.opensslCli))
   exports.opensslCli = false;