Inspect output if test uses temporary directory

Set environment variable TEST_APP_TMP_DIR (e.g. /tmp/electron-builder-test). Specified directory will be used instead of random temporary directory and cleared on each run.

Test Code Signing Ceritificates

If test installer certificate is expired: http://security.stackexchange.com/questions/17909/how-to-create-an-apple-installer-package-signing-certificate