test: unlink custom unix socket before test
test: unlink custom unix socket before test

Unlink NODE_COMMON_PIPE before running the test if set in the
environment.  The test runner won't do it for us like it does
for files in test/tmp.

PR-URL: https://github.com/node-forward/node/pull/40
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

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