test: fix test-debug-port-cluster flakiness

Remove port increment by `1337` which appears to sometimes result in a
port that is already in use. There is no reason not to use
`common.PORT`.

PR-URL: https://github.com/nodejs/node/pull/6769
Fixes: https://github.com/nodejs/node/issues/6754
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-debug-port-cluster.js

@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-const PORT_MIN = common.PORT + 1337;
+const PORT_MIN = common.PORT;
 const PORT_MAX = PORT_MIN + 2;
 
 const args = [