test: run tests in parallel, common improvements

* Allow running tests in mixed parallel/sequential modes
* Add -J flag for running tests on all available CPUs
* Support TEST_THREAD_ID in test/common.js and use it for tmpDir and PORT
* make: use -J flag

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/iojs/io.js/pull/172
Fix: iojs/io.js#139