test: only refresh tmpDir for tests that need it

Expose `common.refreshTmpDir()` and only call it
for tests that use common.tmpDir or common.PIPE.

A positive side effect is the removal of a code
smell where child processes were detected by the
presence of `.send()`. Now each process can decide
for itself if it needs to refresh tmpDir.

PR-URL: https://github.com/nodejs/io.js/pull/1954


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>