tools: use absolute paths in test-npm
tools: use absolute paths in test-npm

Updated test-npm to use absolute paths for tmp/cache/prefix

PR-URL: https://github.com/nodejs/node/pull/3309
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>
Reviewed-By: Jeremiah Senkpiel <f1782d15e31ad502ea2c097c21cc235811d0cb7c@rocketmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node