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>

test/parallel/test-https-truncate.js

@@ -11,8 +11,6 @@ var https = require('https');
 var fs = require('fs');
 var path = require('path');
 
-var resultFile = path.resolve(common.tmpDir, 'result');
-
 var key = fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem');
 var cert = fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem');