Use the T.TempDir method from the testing package to create temporary
directories for tests. Directories created by T.TempDir are automatically
removed when tests complete.
Initial implementation constructed the hash input incorrectly. New test uses a key modulus that is actually on the openssl weak key list instead of a random placeholder.
Adds a basic truncated modulus hash check for RSA keys that can be used to check keys against the Debian `{openssl,openssh,openvpn}-blacklist` lists of weak keys generated during the [Debian weak key incident](https://wiki.debian.org/SSLkeys).
Testing is gated on adding a new configuration key to the WFE, RA, and CA configs which contains the path to a directory which should contain the weak key lists.
Fixes#157.
Eng Zer Jun