test: make temp path customizable

In CI we previously passed `NODE_COMMON_PIPE` to the test runner to
avoid long filenames. Add an option to the test runner that allows the
user to change the temporary directory instead. This also allows us to
run test suites in parallel since `NODE_COMMON_PIPE` otherwise would
have been used from multiple tests at the same time.

PR-URL: https://github.com/nodejs/node/pull/3325


Reviewed-By: Joao Reis <reis@janeasystems.com>